I have similar size listening room in terms of volume; my listening position is about 12 feet from the main speakers. I regularly listen at levels between 95 - 100 dB.
With M80 as main speakers, and an amp putting up 300W (all channels driven), I have a lot of power to spare. My preamp volume goes about half-way to reach that sound level.

I would say 200W on all channels should do, unless you have very, very inefficient speakers or you sit 20 feet away from your speakers.

M80 v3, 2 x Emotiva UPA-1 Monoblocks, XPA-5, UMC-1 PrePro
Pro-Ject RPM Genie, Carver C2 Phono Preamp