The Dayton Sub1200 is one of the best options, I think, for a budget Sub. In fact, the performance between the 12 and the 15 stacks in favor of the 1200.
It is limited on the high end, to 150 Hz, iirc, which is possibly the only real drawback if you need to transition from small desktop speakers.
Of course, that is the point in shopping where you have to weigh the tradeoffs.