I have had the M80s going on 4 years now. They are a wonderful speaker for a music system, which is how mine are used. They are in a room that is about the same size as yours and they do a fine job. My M80s are powered buy a Rotel RB 1080 amp (never shut it down either) and they can play at insane levels in my room. As a matter of fact my ears give out long before my amp does.
You should be good to go running the HK 635 with the M80s. With it having pre outs you could always add an amp at a later date if needed or desired.
