That is a huge room.

How well do the dual STF-2's perform?

Anyways, I would keep the seperates. It allows you more flexibility down the road. I also think the 7100 power amp provides more real power than the 2400.

Another option would be get the 2400 and a nice 2 channel power amp for the M80's. The best thing the 2400 has going is YPAO.