I know this is a bit over budget, but
SVS PB10
or
HSU STF-2
are highly recommended ones. Edge going to SVS for HT, HSU for music.
If a matching Klipsch sub fits in your budget, nothing wrong with that (especially if you can get a discount on package pricing).
Another sub in this range to look at would be a JBL e250.
Dual
dayton 12" subs would be another possibility.
How big is the room you're in? You have very efficient speakers so they should do a good job filling even a fairly large room. It would be a shame to not have a sub to keep up.