That's what I was wondering, because I recall Jason/Myrison had a humble sub that the ep800 just blew away. I tend to suspect if I go with another Axiom sub I would end up getting another ep800, which would cost "only" 30% more than adding two ep350s. If I could get something for like $800 that would fill out the low end I might not have to/want to wait so long, but to ensure the same result as what I'm hearing most likely I will end up having to wait.
SVS makes a sub that you can pick up for about $950:
It's not as efficient, and does not go as low as the EP800, but it's a great sub that can get down to 16Hz comfortably. It's gotten good reviews too (I think a couple of members on the board have this sub).
If I were to buy a sub that was not an Axiom sub, I'd probably go with this: