@Darkebear’s response isn’t 5, it’s a mixture of 1 and 2.
The only way to get 5 is to be rich enough to have a dedicated listening room that’s at least 6.5m wide and at least 7.5m long, built with (or reinforced by) heavy and strong materials (and even then it still needs to be furnished appropriately to get the desired damping characteristics - in other words, using the furnishings as a form of acoustic room treatment!).
I would say a mixture of 1 and 5. Because I didn’t see “ I know I don’t care “ but more I prefer without , because he said” if you can’t be involved in the music then room needs attending to”. However DB feels involved.
However he uses some voids behind side curtains as bass traps. He believes his room doesn’t necessitates important acoustic treatment.
That to me seems like some substantial room treatment! - this is solution 1
These indicate that DB accepts the remaining limitations of the room as it now is, with the degree of acoustic room treatment that has been applied - this is solution 2.
Not a true answer. As I explained in an earlier post, bookshelves full of assorted, books with non-aligned spines certainly can and do diffuse sound. They just don’t do it as well/thoroughly/uniformly as purpose-deigned diffusers.
What is the sound source giving those spectra?
You would do far better to get a free copy of REW to run on a computer, and a measuring microphone such as the low cost one recommended by REW, and run some tests with the computer feeding sound (frequency sweeps or pink noise) into your hifi ststem.
I’ve not commented on this thread - but since I’m being quoted from elsewhere, which I don’t mind incidentally, my view on rooms and their management/treatment, from my own experience, is as follows:
There are rooms that begin without treatment as very good, good, ok, poor, awful… you get the idea!
I like to begin with at least an ok room and preferably a good room as basis for installing a working HiFi system.
My definition of ‘working HiFi’ is that it does what I purchased it to do - play good music and keep my attention with the music so I enjoy it and get the sort of personal and sometimes shared musical experiences that are the delight of our hobby.
I find then that careful treatment of the room first as a living space you must be comfortable being in comes first - if the room feels oppressive or overpowering on me I’m less likely to be comfortable in myself and enjoy the music. It is not just Speakers/Room but Listener that should form part of the assessment.
After I’ve tweaked all the things in the room - position of speakers and large objects - listening position - reflective/absorbent/diffractive surfaces - then am I immersed in enjoying my music as renedered by the HiFi?
If yes then further treatments can be nice and are a secondary task to explore ahead.
If no - and I can be sure it is a room effect and not HiFi poor installation or poor system integration, then I take on the room effect more seriously and you get into treatments.
But my own experience with treatment is that is is very easy to go to an extreme and get a dead neutral effect which I personally hate. I like the early reflections from rear wall as linear as possible for example as my brain seems to have in-built millions of years processing to deal with delayed reverb and a clean secondary wavefront is fine but a non-linear one part diffracted, absorbed in some parts of the audio band, seems to confuse the image for me.
Against this the orthogonal floor reflections for me need a good absorbent surface (carpet/rug) or they throw-up muddle and mess into the end result.
So I have a ‘live’ room with the immediate and first reflection from rear wall as clean as possible - then a mix of absorbent and in particular behind the listener, diffractive surfaces. Diffraction/scattering is more linear to my ears. Absorbent surfaces and object - particularly the objects, I have to treat with great care, as some of these have their own character of delayed release of energy which ruins, for me, the spontaneous musical effect I want to have.
So certainly not against careful treatment of a room, but also listen to what you actually achieve subjectively rather than what a spot-reading by instrument may suggest, or worse an average response which includes secondary resonances.
The reason I did not previously comment here as I think it is a can of worms and an Art where someone who has some humility, experience and technique will gain great results. 
DB.
DB, very nicely expressed.
Interesting description of your living/listening environment. Although being slightly more on the “treatment side”, I share your view about what a listening room should be: a comfortable environment to be in, with or without music.
Another great source of information:
Master Handbook of Acoustics
I’ve leafed through it yesterday evening. It is worth the the time spent!
If you google it, you’ll find a PDF of the fifth edition (the whole book!) 
There is a sixth edition.
A review on GIK I found today. Couldn’t copy the link because of commercial links at the end.
May interest some here.
February 2016
GIK 4A Alpha Panel Diffusors
One of the best values to treating any good listening room.
Review By Rick LaFaver
I have always been a staunch advocate of adding room acoustic devices as an integral part of any high quality sound system. That is unless you listen within an anechoic chamber or a padded cell. There are many different ways to handle standing waves whether it is DSP, diffusion, wide spectrum, or narrow spectrum absorption. GIK 4A Alpha Panel Diffusors are a great way to improve your listening room and here’s why: Two of the most important elements in creating the foundation of any great listening room are choosing a world class speaker and an acoustic treatment plan that tames any anomalies in your room of choice. I have used a blend primarily absorption with limited diffusion in my space for many years by using four GIK 244 Bass traps to deal with the rear wave from my speakers. In addition, they are either in two corners that tend to store a lot of energy or at first reflection point on the side walls (when I have my system setup along the short wall of my room.)
I had opportunity to have some time with GIK’s new 4A Alpha Panel diffusors for those that want to add some basic absorption and diffusion into a room without committing the time, resources, and wall space to a deep quadratic diffusion array and larger absorptions panels. The GIK 4A panels are a great start. In some less lively rooms, these may be all you need at the first and second reflection points. The 4A Alpha panels feature a 0.25” thick Maximum Length Sequence diffusors to scatter sound waves down to 1000Hz to tame reflection in the treble range, on top of a panel with an absorption coefficient that peaks a 1.3 at 400Hz with a slope to .65 at 5kHz. This is a great starter kit for anyone looking to deal with the typical acoustic problems of a suburban home with too many reflective surfaces (windows, mirrors, pictures) and excess energy being reflected. These reflections are from the front wall, first and second reflection points (walls and ceiling) that muck up the soundstage with out-of-phase acoustic information (otherwise known as distortion). Not that this single kit of four panels would completely treat a room, yet for only $340 you get four 24”x24” panels that are perfect for treating either first or second reflection points. They can also dampen the reflections in the front of the soundstage. If you only had one kit you could use it to do both, but I would get two kits and put four panels in front and four panels on the side walls. Depending on how much energy storage you are getting on your front wall, you may want to use something more like the GIK 244s. The 4A Alpha panel really shines as a simple, low profile, and elegant way to deal with first and second reflections on the sidewalls. The diffusion isn’t as elaborate as some quadratic diffusors, but is perfect at diffusing the critical treble at those pesky first reflections. Aside from bass modes this can be some of the most distracting distortion characteristics in a room so very important to deal with.
First Impressions
The 4A Alpha Panel comes in nine standard fabric options and nine premium fabric options. The diffusor portion of the panel comes in a pleasant-looking natural maple finish, although it would be nice to have other options on the wood finish to match décor or speaker finishes. They are similar depth to the 244 bass traps at around 4” in depth. The MLS diffusor is a well understood concept and is executed well. Although the frame of the panel would be easy to put together, save for the complications of the panel design and engineering, the fit and finish on the panel and the fabric (throw in the diffusor panel, the CNC cutting, and the MLS diffusor design) at $85 per panel they really are a bargain. GIK also offers different finishes in the coming months. The weight of the smaller panels makes them incredibly easy to manage and mount in your room. The 244 FRT panels have some heft to them, and can be mounted with a screw into a stud, but I mounted mine with two screws and wall anchors (I have a little one that sometimes sneaks in). These are light and feel very secure off a picture hanger and a small nail, they may not even need a stud if you can’t find one convenient for placement.
Sound Improvements With GIK
This is a tough one, as it is easy to listen to the difference with a component CD player, DAC, amplifier, etc. You connect it the way the manufacturer suggests, you use your reference systems and cabling to connect it, listen to a few tracks you are very familiar and a few tracks that make your toes tap you put in the hours and you listen. With an acoustic treatment, a proper one, you are changing your system reference in a significant manner. Within my system I previously used absorption at the reflection points with no obvious acoustical issues. I also used absorption in the front soundstage. For this review I experimented replacing the front and side panels with the 4A panels and leaving the others in place, replacing all four locations and select locations and listening.
The impressions were mixed, taking into account my room is already quite well treated the differences were subtle. I found the strength of the 4A panel was as a world-class treatment for the first reflection points. The addition of attenuation of the high frequencies through the diffusor along with sufficient absorption in the mid bass and slightly less attenuation in the low bass frequencies provided a little bit of extra liveliness in my room and the additional bass also provided grounding for the soundstage giving the perception of a wider soundstage. As a means to deal with the energy in your front soundstage, for which I was originally using a pair of GIK 244 Bass traps, the 4A does not really absorb enough energy. They are far better than no panels at all for the front stage but you don’t really take advantage of the diffusion since treble behaves more linearly and the port doesn’t typically produce any frequencies in the effective diffusion range of these panels so you would probably be better off with something that is more devoted to absorption in the front part of a system. The GIK 4A does provide a good level of absorption just not as much as some panels available from GIK and others. Typically there is a lot of extra energy in the front of you system from port noise and the back wave of the speakers can cause a lot of distortion and generally muddle your sound stage. If your front sounds stage is wide or if you speakers are a long way off the wall with taller ceilings, the 4A may be sufficient, they were slightly lacking in my room for this purpose.
Conclusion
I have generally been very pleased with GIK products and the 4A Alpha Panels are no exception. The fit and finish is exceptional, the choice of fabrics are extensive enough to meet the needs of most rooms, the size and weight make them incredibly versatile to place. In addition, the diffusion adds a useful feature to an already world-class panel. Even though the 4A Alpha panel offers more features and options than most of GIKs offerings, they are also one of the best values! The 4A would make a great first or last addition to treating any good listening room. I found the Alpha panels to be a significant upgrade to the 244 bass traps in my first and second reflection points. A deeper quadratic diffuser would certainly be necessary within a more lively room than mine, yet the diffusion the GIK 4A Alpha panel offers was a noticeable benefit within my listening room. More bass diffusion may be necessary if you find that your side walls are part of your bass management problem. In my room, these positions are not overly critical to control bass anomalies. GIK also offers a variety of finishes and veneers that should work well with virtually any décor.
Bass diffusing panels or bass traps ? In which case choose one vs the other? Only measurements with recording by microphone can answer to that question?
You wrote it several times, I do remember. However I understood that for precise acoustic treatment, careful set up, you need to use a microphone first. I got it.
However my question is a bit different: If you have not well controlled bass, do you need bass trap or diffusing panel? I guess a microphone is not necessary to respond to this global question.
As for bass diffusing panels, I just used this term employed in the review I posted above. Not my term.
I pasted the part of the conclusion of the above review on GIK 4A panel.
So the reviewer says bullshit ?
FR, in order to tackle a problem such as room treatment learning is key!
Reviews aren’t the best way to start.
I posted several information sources, even in french.
A first step would be the video that started the thread.
Easy to understand, no numbers, no calculations, just a guy explaining how it works.
Speakers and electronics, even used, are expensive toys.
Room treatment doesn’t need to be expensive.
So, don’t buy new speakers, don’t buy new electronics, and don’t buy fancy cables… or at least not yet 
For less than $200, and some DIY, you can get a pretty decent result buy treating first reflections.
Unusual and refreshing video:
and here is where to place them:
Or entrust to a genuine expert - e.g. GIK provide such a service free of charge, and they can receive REW measurement files (and, if you ask them, advise what measurements to make). But as their default would be to try to achieve the best practical minimisation of adverse room effects, if that is not what you want you would need to define what you want to achieve.
Absolutely! 
And they ship allover Europe.
Interesting read Thomas. I have had great success with ordering QRDs from Bluetone in Poland. Beautifully finished product and the cheapest out there. ATB Peter
Do you think their bass traps are effective as trip traps from GIK, which cost 3 times each?
