High power amp, does it matter at low volumes?

The first watt matters.  A better low power will give you a better SQ than more power.  The Gaku-on from Audio Note is/was probably the finest sounding amp in existence and is 45 wpc - and was about $200K 15 years ago.

For much of the music I listen to my SE 2A3 amp is gorgeous (a few wpc).  I am currently in an orchestral frame of mind and have put the 50 watt into 4 ohm XA 25 back in, it is cleaner on crescendos, but a hair of the magic is missing on pp voices.

Class A sounds better than AB.

If you are listening at low-medium volumes you are probably not going above that first watt.

Download/stream a test tone (AudioCheck.net and Spotify e.g.) adjust the volume so that it is a loud as you want at your listening seat and put a voltmeter across the speaker terminals.  It will probably measure 2 volts or so.  ( P = V^2/R) => 1/2 watt into 8 ohms.  Your speakers are nominally 90dB @ 2.83 volts so that is 87dB @2v.

I measured a 3 or 4 dB drop from 1 meter to my listening position so the 1/2 watt gives about 83 dB.  Enough to damage hearing after 4 hours.

Per Welcher, I normally listen at a max 65 dB, at my seating position. Mostly rock, blues, some lighter music like Jesse Cook.   

If I understand everything correctly, I am using 1/4 watt of power to get 65 dB at 12 ft ( I added a bit of distance ).  Adding in the dynamic factor for rock, normally what I listen to, guys like Tommy Castro, I get 108 dB spl at the speaker, which works out to 64 watts ( to handle the dynamic range ).

Some fascinating information here.  The 3 dB rule gain for double the power. 6 dB rule for double the distance. I don't remember ever hearing any of this..

So, what I am getting from this discussion is I would need atleast 64 watts ( with that extra distance of 12 ft vs 10 ft ) and I would run my amp at full output, if I bought an amp rated for 65 wpc.  Of course, buying an amp at 120 wpc would mean I would run that amp at half power, a 250 wpc amp at 1/4 power, etc.

I suspect the next factor is running anything at near it full rated load has drawbacks.  Hence, using a high powered amp at lower power is preferable.

I have always thought working the tubes a bit makes them sound better.

Also most potentiometers degrade the sound at lower settings. Stepped attenuator maybe not.

SET amp for low volume.

SET amp will poop itself with B&Ws.
B&Ws need current. It’s not complicated. An amplifier cannot produce current if it doesn’t draw significant amounts of power from the line. It might not be a high wattage amp but if it draws the needed power from the line to produce current required for these speakers you’ll be fine. 
 

If you're listening at low volume, wpc doesn't matter in the least and neither does the type of amplification. Class A, G, D, etc will have no effect on the sound. In fact, even a "flea watt" tube amp will supply enough power to produce low dB music but if you get the urge to crank it, you'll be limited with how loud it will get.