At $2500 for something that looks nice, I would definitely go for dual subs. I would be mostly looking at sealed subs as ported subs tend to be huge. You do sacrifice deep frequency performance by going with sealed though.
Subs brands that I would be looking at is SVS, Hsu, Paradigm Defiance X series, Monoprice Monolith, Rythmik Audio sealed 15"s, Outlaw Ultra-X13 (if it ain't too big). Maybe the Seaton Sound JS-12. Maybe MartinLogan Dynamo 1100X.
The thing is, if you are going to end up with some 804 D3 speakers, there probably isn't a whole lot that sealed subs will contribute to the low end on a rig like that, so it becomes an argument for ported subs which will have much stonger and deeper bass than the 804 speakers. The problem with ported of course, is that they tend to be big. Sealed can make up for that in number though. For around $2500 you could get three Hsu ULS-15s or four SVS SB-2000s. If you place those around the room, you can get a very good frequency response with plenty of output.