The first thing I would do is check the settings, re-calibrate if necessary and also check the placement. I know that sometimes the sub has to go where it currently is sighted due to furnishings, windows, electrical sockets and so on but often moving it can solve the issue. Some folks also recommend isolating the sub using a platform, slab or granite chopping board (I kid you not).
If you do go down the new sub route, I would recommend checking out the BK range or SVS. You'll get a heck of a lot more sub for your buck with anything from those two. They do boom but in only with nice, tight, controlled bass.