I was in this position once apon a time. I decided to go with an EP800 instead of twin 500's. I think if I had it to do over again I would have gone with twin 500's myself. Not because the 800 didn't do a fantastic job, but because you get more options as far as killing bass 'lulls' in the room with two units. And seeing as this is in your basement I would assume you have a lot more creative control over what goes into it than you would upstairs in say the living room, eh?
I don't think you'll be disappointed either way you go, but if it were my money right now, it'd be twin EP500's on their way to my house!
This is the major point IMO. Bass lulls are a definate problem with one sub, I would imagine in a large room it would be even worse.
