The sealed v4 subs are amazing. I can't say that about the v2 because as a lot around here know, I really struggled with the 600v2 to the point where I was going to burn the damned thing in my firepit. Ian being a very generous human being kindly allowed me to trade it in for an 800v4 whose settings I haven't touched since I set it up a year ago and provides me nothing but sheer joy. I don't even think about the 800 the way I was constantly nagged by the 600. It just provides clear, transparent, permeating bass as called for!
As to whether you need two or one, I just don't know. I have one 800 for my 4,200 cu. ft. space and that works very well for all my listening positions. It's at 1/3 gain on the rear knob...LOL...it's over-kill even for my large space. Even with that 1/3 position, Audyssey turned it down by 2dB. No one complains. Not even me when I sit off my MLP and that says a lot because I am very picky. One 500 will easily fill your space.
BTW, I'll just throw this in for your consideration. I had an old EP400, the one with the triangular port (v2?). I placed that in my 4,200 cu. ft space and it was better for music than my 600v2. It filled my space just fine. Movies just didn't have an impact though. When I get my v4 400, I plan on trying it in my large space. The v4 is sealed and it has a re-worked amp that Andrew slaved on for 2 years. I think it's the same amp as the 500/600/800 but it has less storage capacitance. The 400 is about the same price as the 500 but it's smaller of course.
Also you may want to consider wireless. It's the same price. I'm trying to find out from Ian if the wireless is as high fidelity as the wired. It should be but I want to make sure.
P.S. I may have made a mistake going to with the EP800. I think an EP500 would have been just fine. I do like the shaking though...LOL! According to Ian, the 600 is "good enough". The 800 is like the M100...the product that only 1% needs (maybe wants is a better word).
