On the 3dB rule
We did some recording the other night to update the Almost Milwaukie demo. We've added a third guitarist, so we tracked his parts. We also replaced the departed singers' tracks with Mary-Suzanne's.
What I found interesting was watching the mixdown process using Cakewalk and observing how a 1dB change would noticeably change how a track sat in the mix. So that got me to thinking about the conventional wisdom that a 3dB change is "just barely noticeable". Perhaps that's based upon hearing the change in isolation... I don't know.
So, conventional wisdom tells us that a doubling of amp power - all other things being equal - won't make that much of a difference except perhaps in available headroom. To put it another way, don't bother with the 100 watt amp because it's not that much louder than the 50 watt amp. Or 50 vs. 25, whatever...
The point is, there is a difference in how your guitar sits in the mix based on amp power, and it may not be just headroom. I guess it depends on how hard you push the amp. If you play "turn it up to 11" rock 'n roll with no dynamics, then maybe amp power (and speaker sensitivity, and EQ, etc...) makes a difference in how your guitar sits in the live mix. When you're not playing flat-out, then that difference translates into headroom or - if you hit the sweet spot - touch-sensitivity.
So, if the band's volume goes up, a small amp will reach a point where it just has no more to give. You lose the touch-sensitivity and the ability to hang with the band playing clean sounds. If the amp has too much power for the volume of the band, you can't push it into the sweet spot and you have to be very careful about picking technique to keep your guitar from sounding either sterile (no harmonic coloration from the amp) or too forward in the mix (picking too hard).
I guess that's the problem that preamp distortion and master-volume amps tries to solve. Put a variable distortion device in from of an amp with tons of power, and you can get distortion at any volume. Sadly, preamp distortion doesn't have the same complexity as you get from having the preamp, power amp, power supply and speakers all working together. On the other hand, I can't see stocking a stable of similar amps having a range of power levels just to match the playing situation.