Should I go with another 800 or two 500s?
Before diving in, definitely wait until you have the one sub to put down $ for anything else. As Satkartr mentions, one of these bad boys does a very nice job filling your room with sound. If you have a big room and care that the bass is smooth for every seat in the room (as opposed to just being perfect where you and your wife sit and watch), then you'd be a better candidate to add another sub.
When I did my initial room tuning with the EP800 and the EP600, I found that the incremental benefit of having the two subs (+ another less capable sub I've always used in the rear of the room) was very small. I was surprised at how little the EP600 added to the effect when the other two subs had already been equalized to flatten response in the room. Given that, I decided to leave the 600 unplugged and sell it as I just can't justify all that additional money for a very modest improvement in frequency response (and the improvement is only in other seats, not the ones my wife and I sit in). I'd look at the argument the same way from your perspective, just that you haven't yet put the money down on a second sub and I've already invested in my 600.
As for the 800 versus 2 500s, I'd definitely add another 800. You get diminishing returns as you add additional subwoofers, so you're going to get a better incremental result from adding one 800 than from adding 2 500s.
What would I need to add to my 3808 to make that work?
Nothing more than a $2 Y-adapter for the RCA cables leading to the 2 subwoofers.
If you're really obsessed with flat response, look into adding a Behringer Feedback Destroyer (BFD) equalizer to tune the response in your primary listening position to be as flat as possible. It's less than $200 and a lot cheaper than adding a pair of EP500s.
Jason
