In a perfect world, the power amp doesn't color the signal at all, but actually the power amp can have plenty to do with tone. The right power amp can hide a multitude of preamp sins, and the wrong one can make any preamp sound like crap. As far as I'm concerned (don't tell my back or my chiropractor), big (200 watts or greater) tube power amps are far more desireable than pretty much anything else out there, tonewise, but are hard to find, weigh a ton and cost a bundle. They tend to do really nice things to whatever signal you put through them. They have very musical compression and the *right* kind of harmonic distortion (I don't remember if it's even or odd-order, but it's the one I want to have) and tend to breathe, for lack of a better term. Crown solid-state amps have always had a similar reputation. Peavey amps have always seemed rather cold, but more reliable than dirt. QSC has an excellent reputation. If you really want to judge the relative value of one type of power amp over another, find and try buying an old McIntosh power amp. People are more inclined to sell you their children and let you date their wife than get rid of those wonderful things, so what they do to a signal must be pretty good. It seems like I recall Rami waxing nostalgic about his old McIntosh stuff, if that's any indicator. And heck, I used to know a guy at Producer's Workshop that could reliably hear the difference between gold and switchcraft patch panel connectors, so pretty much everything contributes (or subtracts)!
John - rambling as usual