some people say different amps color the sound differently. I'm not quite sure how this happens.. this might be like saying black cables have the darkest sound and yellow cables have the brightest sound...
but then again, I might be wrong. I just always had the impression that power is power, and it doesn't take the analog sound coming in, alter it, then power it out to the speaker. I think the other electronics have more to do with this.
One myth to beware of is false power ratings. Cheapo sony or rca might claim 200 watts, but these are peak ratings at a low impedence level. A good company like Denon, HK, Onkyo, and Yamaha will offer lower numbers, like 70 watts per channel, but they are a real 70 wpc RMS, which means continuous, not peak.
