Three notes/Three caveats:
Caveat #1: Your budget is sufficient to buy 2 very high quality "small" subs - call that +/- $1500.
Note #1: A single large sub will usually provide great maximum output for a given distortion level than will two smaller subs. This observation is based on pretty extensive review of subwoofer test data.
Note #2: You can (in the vast majority of cases) achieve smoother bass response with 2 carefully placed subs (large or small) than you can with a single sub (large or small).
Caveat #2: If you're using digital room correction (e.g. Audyssey) in an AVR or Pre-Pro, you can smooth FR with the EQ.
Note #3: If you cross high enough you will need to maintain stereo integrity so 2 subs will be required.
Caveat #3: IME, few people will cross to the subs at a high enough frequency (>100ish hz) to make this concern audible (to me). Others claim that the effect is audible at a lower frequency, so this is a YMMV kind of deal.
Marty
PS If you wish to go with 2 subs for purposes of maintaining stereo integrity, make sure that your x-over will allow this. To my knowledge, very few AVRs or Pre-Pros (even those with dual sub outputs) are set up this way.