Preamp + power amp (100w) and very efficient 93 dB speakers. In this case listening at low and medium volumes power amp will use extremely low power, for example 0.2-0.5w or so.
Preamp + power amp (100w) and not efficient 86 dB speakers. In this case listening at low and medium volumes power amp will use 2-5w or so.
In which case power amp will produce better sound quality? I want to look into these nuances before buying new system. I think they are very important, because most of the time we listen to music at low volume levels. I understand that speakers also play big role concerning low volume listening, but for now my question is only about power amps.
Do power amps have some kind of “sound quality vs. power used” curve?
The answer will depend on the power amp, however any hiss from the power amp itself, or the output stage of the preamp (or hum, but that less likely to be present) will be louder with the more efficient speakers, so the lower the listening level the more likely to be audible during quiet passages. Generally speaking very high sensitivity speakers require low noise amplifier designs if used to play quietly. Against that, a great advantage of efficient speakers is being able to achieve realistic high peak levels on high dynamic range material when you do want to play louder, without pushing an amp into clipping, with associated reduction in sound quality and increased risk of speaker damage.
Incidentally, your figures aren’t quite right, the speakers would need to be 10dB difference in sensitivity for 10 fold difference in power for the same listening level.
However your 2-5w with an 86dB speaker is not particularly quiet, at least, not if you are referring to average power, as opposed to peaks, though it is dependent upon how far the listening position is from the speakers: At, say, 3m from a stereo pair that would be nominally 82.5-86.5dB average level - not extremely loud, but not quiet either. Quiet listening might be at least 10dB lower (0.2-0.5W with the 86dB speakers at 3m).
As an single measurement, sensitivity as a number means not so much sadly. At varies across the spectrum. I remember moving from a pair of speakers to a new pair that were lower sensitivity… and yet they were in reality much easier to drive. I had to significantly throttle back the volume control from my previous levels.
I would recommend greatly against buying on numbers if you can. I know not all countries have the same quality of local dealers and demonstration facilities so it isn’t always possible to buy using your ears but make every effort if you can.
As to your questions about quality vs. power. Yes and no. In terms of power amp design, it gets harder to deliver quality with each additional watt. But that is in terms of absolute output capability. Any amp driven within its comfort zone should be fairly consistent. Whether the speakers and room interactions hold up their end of the bargain as volume increases is another matter entirely.
For this reason, there are more low powered 25-30w amps on the market than 100w+ amps. People spending $10k on 25w valve amps aren’t necessarily doing low level listening, but they are using speakers that are very very easy to drive.
Anyways, at 100w, you can pretty much choose whatever speaker you like that sounds good to you. Your compatibility window is fairly large I imagine. Also, only you know what “low level” means to you. I had a friend who’s idea of low level was louder than when I crank up my system. Conversely, when my wife talks about “low level” listening, I always tell her to just turn the system off because at that low volume I don’t see the point of even playing music. It’s all highly subjective.
From my recent experience of testing different combinations I have concluded that how well a system performs at low volumes does depend on the overall interaction between the components, including DAC, Preamp, poweramp interconnects and speaker cables. I reckon this can only be validated through home trials.
For example I have found that introducing an Entreq Primer Pro between my new Hugo TT2 DAC/Pre-amp and mononloc power amps (145w per channel) and then Tellurium Q Black II speaker cables has transformed the ability of my mixed system (Naim front end of ND5 XS2 and Unit Core) to deliver a satisfying sound at low volumes into my ATC SCM11’s. With more conventional Chord Shawline, which were fine at medium volumes, this was not the case.
You could look into power amps that run in Class A for the first few watts and then into Class AB for more.
Most of today’s better amps avoid crossover distortion - or mask over its effect.
Across the board intelligibility at low volumes is system dependent. Support, cables, isolation, set up etc.
As Naim pre’s sound much better the higher up the volume knob goes, least efficient speakers would be best. I have played around with this myself as I listen very quietly late at night. In my case, I have attenuated my source even further using 9 o’clock on the volume dial for best results. Anything below 8 and I notice a huge difference, and also run into balance issues.