room dimensions 13.5 x 21 x 7.5

which is the best way to go ? and why

How many seats/listeners?

Other than the obvious characteristic, physical size, two subs can be advantageous over one to provide smoother lfe throughout the room. But, the 800 digs deeper, so you would loose some of the ultra low lfe tones with the 500's. Really, in your room, one 500 would be more than adequate.

Seems like this question comes up a lot. I still think Axiom needs an FAQ section.

Troy, unless the few Hz lower extension of the EP800 would be significant to you, two EP500s would be the way to go, in my view. The two EP500s would have at least as much maximum volume capability, and properly placed(basically at the midpoints of the opposite side walls or front and back walls or in diagonally opposite corners)they would smooth the overall bass response by reducing the effect of room modes.

Tomtuttle:
there will be three seats one row 11ft away from the front speakers, and probably just me sitting in the middle.

Thanks

Yes I think two EP500 would be more than enough.
Just wasn't sure if I has the dual EP800 if it was the same as two EP500's

Some time ago, I was debating on a 500 to go with my 600 but ended up with 2 350's. The combination of those up front and the 600 in the back are killer, especially in the new enclosed theater. I know I know where are the pics.

The 800 will give more impact on movies. The dual 500s may give a flatter response spread over a wider viewing/listening area

What Fred, JohnK and Mike said.

Personally, I'd start with one EP500 and see how that goes. If you feel like you need more output, decide from there. I think the EP800 is an awfully big enclosure.