Or sell your current one and buy a bigger, badder one altogether. However, subs are the perfect first time diy endeavor if that's an option and/or of any interest. (A driver + an amp + a box). But if you want "impact" for "HT transients" from the aftermarket crowd something like the Sunfire might be more up your alley (if your bank account will permit).
2 subs will give you the luxury of stereo bass (whether or not is audible depends on the room somewhat) and it will help smooth out frequency response--both good things. But you seem to be more interested in the output/spl department. In an article in Audio years ago there was an experiment with the use of two subs and what best way to position them within a variety of rooms in relation to stereo bass audibility and spl levels attained. The author's findings, and its been along time since I've read it and I don't know where the article is so I may be a little wrong, was that 1) within there tests with pure test tones down in the deep bass territory, no one could reliably localize the sounds origin (now if your room is 70feet wide its different story) 2) they got the greatest output in most of their rooms placing both subs in one (the same) corner. Someone else may know of something new, more academic, or definitive on the stereo bass issue. But if you mains can go pretty low, one better sub might give you everything 1)low end ooomph 2)the stereo thing won't be an issue. (I can't recall the article addressing room nodes and multiple subs.)
I used to own the SW2P, it wasn't much--the amp was pretty cheap having inspected it, but the cabinets were pretty dense. Obviously if you buy another sub for stereo it'll be and SW2P, I couldn't see running two different models of subs in one room.
