A “trick” for finding good placement for your sub is to initially put it in your listening position, and move around the room yourself. Much easier than the other way around. Once you find a good spot, switch yourself and the sub around to finetune.
How do you define a “good” spot?
Where you like the sound / room interaction.
Buy a REL…
I’m going to suggest that, beyond the procedural aspects of proper sub placement and system integration, why are the 23s considered to be deficient in bass response in this case? That would be my primary mystery to resolve.
Good evening sir. I’ve just looked at all of the comments and didn’t expect it to be channelled the way it has. I was asking a simple question and thought I would get an array of people’s comments relating to their own personal experiences. I am grateful for the great advice given btw. David, I’m not used to such small drivers and when I spoke to the specialists a few of them mentioned a sub being an option. If you’re telling me that these speakers can replicate the depth and the force of 4, 6 inch drivers then thank you for making me aware of that… that was the purpose of my initial post. 
@Errol Still don’t really get it…Was under the impression that you owned the PMCs but were dissatisfied due to lack of bass response. Your latest post seems to indicate that you’re wondering if the PMC 23´s 140mm driver using TL output will be sufficient. Only you can judge that with the speakers in your room, properly amplified and broken in. Several other posters have responded that these speakers provide sufficient bass response in their rooms. They are rated to 30hz. Will be curious to learn where you end up on this question as I’d be interested in the new 23i but…not if they’d require a sub. And no, I do not think that two 5,5 inch drivers will equate to four 6 inch drivers. The law of physics explains that one rather quickly. That doesn’t mean that the PMC won’t provide satisfactory sound when coupled to your room. Try a home demo and see…
That was a quick reply. David due to the current COVID19 situation I am unable to go to any Hi-Fi shops and have a demonstration. That was the reason I asked…
I have now purchased some 20-23’s and they should arrive on Monday. I will most definitely be adding my thoughts on them Monday evening. The speakers might have full resolution but can they also be fun 
Monday can’t come soon enough.
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I have been following this thread with interest. Two reasons, one being that I have added a sub to my system and am 90% satisfied. It currently sits in a domestically acceptable position and I know from putting dimensions etc into REW there may be a better place. I might be asking for further help at another time.
Then it is the start of the thread, the apparent lack of bass, 1 the PMC produce an extra octave of base compared to my Graham LS5/9, except going to the PMC website the specs say 29Hz, but not the limits, flat? Plus or minus 3dB or 6dB? Many moons ago I used a kit speaker, HFN/Fris Daline, effectively similar to the twenty 23, a transmission line using a Kef B110, a 4 inch diameter bass driver. Using a Nytech CTA252XD they would shake the room. A friend who had access to test equipment assured me they were producing down to 18Hz in my 24’ 7" by 11’ 10" room.
So I totally get what DavidParis is saying. Yes, a good big 'un may always beat a good little 'un but in this case I cannot understand an apparent lack of bass.
Finally, if considering what sub to buy, check out the history of BKE, they may be cheaper, but that doesn’t mean they are not superb units.
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Looking forward to your opinion! Thanks.
If the mid-range sensitivity of the speaker system is 3dB less than the system with 4x150mm drivers and the throw (i.e. the distance the cone can travel back & forth) of the 140mm drivers is 15% longer, then the 2x140mm drivers can match the system with 4x150mm drivers.
(The above figures are just an example, other combinations of increased driver throw and lower overall sensitivity can also be made to work equally well.)
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Errol,
I purchased a pair of 25.26 about 7 months ago. Due to change in work schedule, unfortunately I have not had a chance to listen much at all.
I finally had some time in the past month, and tracked the hours I was listening to them for break in. After some tweaking for placement I finally got the bass where I wanted it for my room and sound preference. For example, I had to get my left speaker closer to the corner of the room to get my bass.
What i soon found out was at about 130 hours of listening time, the bass really came on, along with mids and treble. A little toe in and minor tweaking and the speakers are really on song. I realize you are acquiring 25.23’s, but you should be just fine without the addition of a sub. I have heard 25.23 in a 5m x 7m room with no sub and they were fantastic.
To summarize, be patient, let them break in, and they are pretty easy to place. Take the time to tweak them in, and remember room treatments and layout are key. You will NOT be disappointed the least with PMC’s ability to deliver wonderful sound, they really are very nice. I have my listening position and distance between speakers in a triangle (mine is 3.7m almost equilateral) as per the instructions given. My toe in amount was determined by by sound stage preference.
Good luck
Scott
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Please read my last reply to Davidparis. I didn’t say that I was experiencing it. I was asking for advice before my purchase.
Errol, I’m sorry if I offended, I honestly didn’t see your reply, possibly due to the late hour, taking best part of an hour to compose, re-write and click on reply.
Part that I missed out, possibly related to the reply by @Xanthe about the throw of the cones and explained in the article about Daline design, a small cone is lighter, in theory accelerates faster (although that might be complicated by having to move further) so potentially gives a faster tighter bass. So even though two boxes may both produce say 30Hz the mass of the cone may affect the sound.
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That sounds encouraging for the 23s. Hoping the new 23i will have good performance as well.
@Errol Hello, if you took delivery of your PMCs yesterday, have you had a chance to listen a bit? Interested in your feedback…
Brgds.
Bump!
We are in a state of suspense here😁
Scott
@anon64018693 @334578
Good afternoon. Sorry i have been busy trying to organise my finances so not had much time to listen but i did for a couple of hours. After my audition i can honestly say that they are very detailed and i definitely picked out things that i didn’t hear listening to my MA’s.
NOW… the problem is that they’re too polite when it comes to delivering the bass. The drive could be better. They do go low…ish but you cannot complain due to the size of the mid bass driver being so small. If i am honest, i think the shape and design of my room has a lot to do with it.
To conclude, for the time being, i am going to keep these babies unless i get a good offer for them and go down the sub route. Implemented correctly a sub adds a whole new dimension to the overall presentation and for those that love dance, reggae, hip hop etc will definitely make you get up and dance. 
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Sounds like overall you are please unless you have opportunity to send them off, are they new, or previously enjoyed by another user? Tweak them a bit, can you post a pic of you listening room?
Scott
Whether this is relevant or not…I had a pair of PMC 20-23. Enjoyed them a lot. I also used them with s REL Strata 3? Which I had from new to compensate for lack of bass with a pair of Nest Mysteque 2’s.
I quickly sold the REL. Not required with the PMCs. And then finally I pxd the 20-23 for a pair of new 20-26s.
Yes there was a bit more bass. But not as you may expect.
The main reason for perhaps not as much bass (change) as you may expect was my power amp.
It was, and is a MF A300cr heavily reworked. This amp is nigh on impossible to clip. It delivers tight bass whatever it is connected to. I even paired it with s cheap/modest pair of £100 stand mounts, and they sounded waaaay better than they should have.
So, maybe it’s not always s the speakers problem. Make sure you power amp is more than capable.