Unless you have a really large room it's not likely that you would need more power than the 635 can put out. Having said that, there are a few people running their M80s with even bigger amps (generally more than 200wpc) because they do have large rooms and play really loudly.

Strictly speaking I guess there are three "regions" your system could fall into :

- playing at the loudest volume I want the amp is not even working hard

- playing at the loudest volume I want there are occasional peaks where the amp hits the limit -- not clipping badly but just a tiny "edge" on the sound

- playing at the loudest volume I want the amp clearly doesn't cut it, I had to turn it down because it was obviously clipping and could hurt the speakers

If you are in the first zone then no problem. If you are in the third zone you know what to do. The rest of us anguish over whether we just might be in that second zone and some people buy bigger amps to find out for sure.

Unless your previous electronics involved a big honkin' power amp I don't think you need to worry. I know my 630 and M60s together don't even seem to be close to the limit when playing as loud as I would ever want.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8