Ben, do you have any idea what brand of amps you would like to go with? Are you going to want to stick with the same series of amps?
I am currently using a 2 channel amp, and a 3 channel. I like this setup, due to the separate power supplies. My 2 channel is powering my M60's, and then the center powers the center and surrounds. With this combination, i have not had any lack of power issues. When i had a single 5 channel marantz MM9000 during some movies a couple of the amp channels would be driven into the clipping region. Since going to seperate amps, i have not had that problem.. However, i will say the new amp channels are over 2X as powerful than the Marantz.
Spearitsound, and accerrories4less are both good places to get a good value.
I'm completely open to brands. My initial thought was to use the AVR to power everything except the M80s which is why I said I may eventually look for a 2 channel amp (possibly 3 channel if I include the VP180). Again that's mostly because it would be less expensive than a 5 channel. Nothing is set in stone at this point except it won't be happening in the short future.
I have had a chance to play the M80s by themselves at about the max the HK can drive them and I don't have any doubts a separate amp could help out a little bit. When I say help out I'm talking about the entire range of freq, not just the low end. I also think some kind of room treatments would help me out but I really need to spend some time learning more about that subject.