Well, I'm no expert on subs, but I do like my EP350. I assume the EP175 would be good for your space and probably integrate well with M3s sonically and visually.
The big subs that are commonly recommended for HT would probably all be overkill for your needs. Rather than worry too much about which sub you buy, you should just try to set-up the sub you do buy properly.
If I were you, I'd get M3s and an EP175 in a high-gloss fancy finish. Or the HSU VTF2 in a similarly fancy finish.
Happy shopping.