Ok...two different questions

What specs/features make amps "sound better?"
So I'm trying to get into the specs of amps and I don't really know what I should be looking at...I'm gonna be using mine for speakers and want to get the full potential out of them.

People are telling me that it's not important to give good speakers a lot of power but that it is important to give them clean power.

What makes one amp's power "cleaner" than another one?

Are there speaker amps vs. sub amps?
Okay, obviously mono amps can't be used for component speakers but is there anything beyond that?

For example, if I had a 2 x 100W amp, I could use each channel for a set of speakers or I could bridge the two and use the amp for a sub...and yet I assume that some bridged 2ch amp would be better for front speakers and some would be better for subs...

Then again, this also relates back to the first question.

Okay, let me know if you guys have any opinions on this...it's a very general question that I'm sure a lot of other newbies need answered too before they go out spending thousand of dollars on an amp without any idea of what makes it so expensive...