It means that a frequency of 31Hz has been captured. In terms of perceived volume it’s low.
A whisper is around -30 dB
(remember it’s a logarithmic scale)
Does the -48 dB mean 48dB lower than the highest sound level?
Not exactly (dB is relative unit).
Let’s put it differently.
-
Normal listening volume : -70dB to -85db
-
Above -90dB it’s loud
-
Under -30 dB it’s silence (perceived silence, of course)
I think you mean a tautology?! 
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To go back to the first question, my view is not yet, as has been said start at the beginning.
Reading the Stereophile review of your speakers is glowing.
Having heard Talk Electronics/Edward’s Audio demos, they make nice music. Looking at the website the cartridge appears to be the same as a Rega Carbon or Audio Technica AT 3600 a budget model with a conical stylus. So to me when people are saying source first, this is the starting point. Reading the Hifi Choice review of the turntable, Edward’s Audio recommend a Goldring 2100, so straight away you are jumping from a £30 to £195.
You have not indicated a budget, but these two figures might give a ballpark figure for getting more information from the LP.
Then you have an unspecified Rega phono amp, could this be the next step?
This may give you the micro details, the clues that support the argument that you can get good bass from smaller speakers.
Can a sub work? For me it does, I don’t know if you can get them to you, but my BKE 200 sealed enclosure is fast, articulate a set so it gives just a little support, tuneful.
At that stage, you get into the room argument and when motivation returns, I too might get round to REW and asking @Xanthe for help.
-48dB means 48dB lower than a reference level. You need to know what the reference level is for it to be useful. The numbers are often used very loosely; it can get very confusing if it’s not explicitly stated!
0dB SPL (ie sound pressure level) is defined as the absolute threshold of hearing. I listen at ~ 70 to 75dBA, measured using an iPad app. With nothing playing, the room is about 33dBA.
The iPad app is accurate enough here - repeatability is more important.
I don’t think those negative signs should be there in your statement of normal sound levels! They are relative to 0dB and are positive values (-90dB is quieter than -30dB whatever it is referenced against!)
Right! Thanks.
Doing/thinking too many things at the same time doesn’t help… abviously ! 
Looking at frequency range where does bass or low range become mid range and where does mid become high?
For which room sizes are 22s recommended?
- Bass would be under 200-250 Hz
- Lower mids from 250 to around 550 Hz
- Upper mids from 600 to 2kHz
- Highs above that
Roughly
I mean modes - spell checker got me !
A thought, no opinion:
My friend has a dedicated listening room in an old mansion. Huge room, enormously heigh ceiling, great acoustics. Bi-amped with Musical Fidelity his old B&W 801 matrix. Listening over there: sound stage, lows, unbelievably beautiful emerging in the music. I love jazz, fusion, pronounced sounds, double bass, rythm…timing…I like my Naim system better
Could it be, room size. Or maybe, Naim is not for you?
For instance, this album, lots of highs and lows, I love it on my system. I tend to stand up and walk further away though, the bass gets more stage presence so to say.
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Roughly as @Thomas said. There’s no hard distinction!
And of course other than a synthesiser playing a sinewave all instruments have harmonics, so where the fundamental of a 4-string bass guitar on standard tuning is 441 to between 300 & 400 Hz, you can still hear it being played even on a very basic pocket radio with a 2 inch speaker, still sounding different from the higher pitched guitar because a guitar’s harmonic are different, going higher still.
I’m certainly no expert when it comes to the technical stuff but I know what I like. A recorded double bass has to have depth but it must be tight, controlled and articulate. It has to sound like a wooden instrument with strings being plucked by fingers rather than the rumble of a passing truck.
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@Airdavid Not sure if there’s a specific room size for them. My room is really small, as in 2.8 x 3.5m. I emailed PMC and explained the size and that they’d be used as near field speakers and they said they’d be ideal. I have no issues all with bass boom at all and it sounds very tight and detailed. If you have a very large room then a demo would be needed think… or, try their floor standing speakers.
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It’s worth bearing in mind - all other things being equal - that a reflex loaded cabinet will likely have a lower -3dB cutoff, but a sealed box will have a gentler slope. Paradoxically, the sealed box may have more very low bass.
The sealed box will also have lower group delay, so is often preferred for mixing and mastering when you want to know what is really going on down there.
-3dB seems to be the (almost) universal standard for specifying frequency range; but if it’s not stated it might be because the manufacturer is being economical with the truth…
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That plot shows about 13dB down at 30Hz relative to 100Hz. As it is a logarithmic scale, 13dB down actually means it would sound as if the power at 30Hz Is only 1/20th of the power at 100Hz. You can still hear it, but it sounds a fair bit quieter. Looking at it another way, 6dB is a doubling of amplitude (loudness), so 13dB down would be a bit quieter then 1/4 of the loudness.
Absolutely. And you should be able to hear the lowest note at the same relative loudness as if you were there in front of the instrument being plucked or bowed. So system, including speaker and room, needs to produce sufficient undiminished depth at the listening position, but well controlled, without evident distortion or overhang.