I used to own a STF2, terrific little sub, it might do well for you. If you are sitting in the center of the room, it might struggle a bit, but if you are sitting against a wall or surface, it should do fine. I would still pick that over the Infinity or Emotiva sub.
Bass gathers along a room's boundaries, and if you want bass while seated in the center of a room, you need to have enough air displacement to pressurize the entire room, as the center of the room will be where the bass is weakest. On your budget, I don't think that can be done, so for you it's all about subwoofer placement with respect to seating positions. If you go with Hsu, they will assist you with finding the best place, just send them a sketch or a description of your room, these guys really understand the acoustics of bass.