Higher power=better sound at low volume?

It all comes down to the speaker - no point in discussing amps. I have a pair of Triangle Comete ES speakers and they rock at half the volume of my JM Labs. If you want low-volume listening, get some forward speakers with excellent microdynamics and mate just about any amp you want to them.

All other things being nearly equal ( often the case in SS amps ), 200 watts power is usually better than 100 watts as it will be less likely to clip as source material becomes more challenging. A doubling of power is usually required to make a perceptible audible improvement.

Transients in music can be challenging even if the average sound level is quite low...this is why a more powerful amplifier or an amplifier with more transient headroom can be better even at low levels.

An inefficient speaker with a difficult and bumpy impedance curve can also complicate matters. Amplifcation needs to be appropriate for your speakers. Damping factor is usally higher in bigger amplifiers...this will better control your drivers and reduce distortion.

A small amplifier when overdriven can destroy speakers many times the rating of the amplifier.

How well it sounds at low volume is what I have been going by for decades.

Speaker efficiency must play a part. If you get 90 db from your first watt and 93 from the second watt, 96 from the 4th watt and so forth, how much power will be needed for low level listening? So I agree with Elizabeth. My speakers are 101 db efficient and sound very dynamic even when played quietly.