My EP500 blows the walls of of a 24x12x8 room (I know sucky dimensions) if you let it. So a 350 should be plenty.
Two smaller subs to smooth out the null spots might be an interesting option as well.
An ep 350 would be more than enough for that room. It has no trouble filling my main space of 20' by 20' with plenty of headroom.
Back at V2 I pegged the EP175 as a much better alternative than HTIB subs, but only HT worthy if you wanted something really small.
Pay attention to that -10 db number. With decent room gain, you should get almost flat response down to that point. Past that, it will drop off quickly.
So, for the EP350 the -10db point is 22 Hz. In my concrete bunker I get about as much room gain as you can and my EP350 is flat to 20Hz.
The 175 will get you down to around 24 Hz. flat depending on room gain. When I listened to the V2 at Axiom it was not quite enough for movies, where the 350 most definitely was.