You're gonna invite us all over to see the place when it's done, right? It sounds very nice!
I just want to touch upon one aspect that hasn't ben answered yet: You were concerned about the WPC of your receiver, and I don't think it will be an issue. The HK will do fine pushing the 4 ohm 80's by many accounts here and evidently, HK's ratings are pretty solid. There is not a whole lot of volume difference between 50 WPC and 100 WPC. And the M80s are your most efficient choice.
My opinion is that if you are in the same space in a few years and could move up to some serious power such as 200 WPC, you might pick up some increased "attack" on some transients when you "open 'er up" in addition to the louder overall volume.
You know, there was a post in these forums not long ago (John?) that makes me feel hesitant to recommend anything I haven't personally heard or done....(!)...but my feelings are that the receiver will do fine with the M80s, and the M80s would be your best choice in a room that big.
Let us know when the party is!
