Does the room look something like this ?
If so, and if the dimensions are accurate, your biggest challenge is going to be a response peak around 56 Hz, caused by the fact that the height (10 feet) is exactly twice the length (20 feet). In order to tame that 56 Hz peak, you will want the sub ~5 feet from one of the side walls, and ideally from two side walls.
I marked a few possible locations. #3 only works if you have a couch or seat in approximately that location and can put the sub behind the seating. If I were you I would start by prewiring for at least #1 and #2, since with a reasonably long cable you can also cover #4 and #5. If the seating fits, #3 might be best of all if the response peak turns out to be really annoying and if you're not into buying equalizers.
Does anyone know if a Denon receiver manual EQ has a control arount 60 Hz ?
EDIT -- first thing I should mention is that placing the sub to correct for room peaks normally works, but doesn't seem to be working for Randy (sirquack). We don't know why yet, so don't be holding your breath for perfect response the first time you fire up the system
re: 2x EP350 vs. 1x EP500 -- I don't *think* this is going to be the kind of "problem room" where multiple subs are going to be required, since one of your offending dimensions is vertical and the other horizontal. Some bass trapping will probably be more effective than multiple less expensive subs. Given that, I would definitely argue for a 500 over 2x 350.
BTW, before you ask -- yes hanging the sub 5 feet off the ground would probably help smooth out your room responses
