You can use the Klipsch as rears the QS8's are better for the side surround duties, the question is can you handle the tonal differences on rear pans across the back, if it bothers you then you would need to get some M2s or more QS8's or similar, otherwise you would be good to go.
Your Denon is more than capable of running the M80s, I ran mine off an 1804 and had no problems what so ever. In a room of your size you could even run M22s and upgrade your sub to have more impact for HT as the M22+ sub is very good, nearly as good as M60/M80+sub. I still prefer my M80s+sub to M22+sub, but that is me; and I have both M80s and M22 around the house to continually subject myself to listening tests to assure myself I made the right decision in purchasing the M80s,
You can also look to the Factory Outlet to save a few more dollars, 10% off list price as well as another 5% for the HT discount which is any 5 products.
