This is a can of worms destined for long discussion, I am sure... My quick take, all IMO:
1. All else equal, two amps of different rating operating at the same power level well within their means will sound the same.
a. Things are rarely completely equal, with things like power supply capacity and charge storage, noise and bandwidth, output impedance, etc. introducing various degrees of difference. Audibility debated endlessly. I find some changes clearly audible, others, eh, not so much. There are also design choices, of course. A tube amp will not sound like a SS amp, for instance. Most of the time.
b. IF you are approaching clipping on the lower-wattage amp, the higher-wattage amp should sound a little better if it does not clip. In the real world, it takes a lot of extra power to buy any significant headroom relative to signal peaks so this is rarely an issue.
2. There are SPL calculators on the 'net you can use to estimate the power you need.
a. Note 3 dB extra volume is a slight increase to your ears but requires 2x the power. 10 dB sounds twice as loud (in the midrange) but requires 10x the power.
b. Headroom to avoid most all clipping is 15 to 30 dB above the average level, or power factor of from 50 to 1000. Note that we tend to reject (not hear) fast (short) clipping events.
c. The difference between 75 W and 100 W is not terribly significant (2.5 dB) but you should use one of the claculators to see what your average and peak needs may be.
3. As you go up the line, most manufacturers offer better amplifiers and larger power supplies. The benefits may not be reflected in the specs. And, of course, you mmay or may not hear any difference.
a. Some AVRs do not put out anywhere near full power with all channels driven, indicating a very weak power supply. Denon is one of the better in this regard, maitaining near full power; my old Sony Elite, one of the worst.
b. Since you only need the power for peaks, in the real world it matters less than on the bench.
HTH - Don
