On your budget, you could get a PB12 NSD or VTF3 now, and if it isn't enough for you, get another. Or you could get one big sub like a Rythmik FV15, Hsu VTF15h, SVS PC12 Plus, or PSA XV30. Alternatively, you could get two Hsu VTF2s or Outlaw Audio LFM-1 Pluses now and stay under-budget while enjoying the benefits of a multi-sub system (better room response and no sub localization). The VTF2 and LFM Plus are pretty powerful subs themselves, probably only a couple DB under the VTF3's max output and a few hertz short of its extension. A system like that might be more appropriate for a room with a broad seating area.