That is a great question. Used to be (in the "good old days") that receivers, amps, etc. had a pretty standard power rating system (X watts into 8 ohms all channels driven). Today, harder to gather from the ads and you really do have to go to the manufacturer's site to look at the specific specs. From the Pioneer site for my bedroom receiver (Pioneer LX101) the site gives these reports:

140 W/ch (6 ohms, 1 kHz, THD 0.9 %, 1ch Driven FTC)
80 W/ch (8 ohms, 20 Hz-20 kHz, THD 0.08 %, 2ch Driven FTC)

When you see an amplifier wattage rating of 100 watts-per-channel at 1 KHz with one channel driven, the real-world wattage output when all 5 or 7 channels are operating at the same time across all frequencies will be lower, possibly as much as 30 or 40% lower.

A better indicator is to base the measurement when two channels are driven, and use 20Hz to 20kHz tones as opposed to 1KHz. However, that still doesn't fully take into account the amplifier's power output capability when all channels are driven.

Compare the specifics with other products and find the common denominator. That's the best way for me to make a determination of receiver/amp power.

I am no expert by a long shot, but it is more than amperage that goes into the issue of sound quality. The effect of amplifier power is demonstrated by this excerpt I found online:

Sound levels are measured in Decibels (db). Our ears detect differences in volume level in a non-linear fashion. Ears become less sensitive to sound as it increases. Decibels are a logarithmic scale of relative loudness. A difference of approximately 1 db a minimum perceptible change in volume, 3 db is a moderate change in volume, and about 10 db is an approximate perceived doubling of volume.
To give you an idea of how this relates to real-world situations the following examples are listed:
0 db is the threshold of hearing
Whisper: 15-25 dB
Background noise: about 35 dB
Normal home or office background: 40-60 dB
Normal speaking voice: 65-70 dB
Orchestral climax: 105 dB
Live Rock music: 120 dB+
Pain Threshold: 130 dB
Jet aircraft: 140-180 dB

Other features such as THD (total harmonic distortion) can go into the equation, although on most modern equipment I think the THD is so low as to be negligible.

Hope that helps! Good luck.