Some of the least likely ones turn out to be the most cost-effective. Cable Elevators, for example. Read my Moab thread, the last ones here heard the improvement easily.
Another recommendation, also for the long term, is a DBA. Don't buy "a" sub, buy 4 of them. In your room especially, the smaller and worse the room the bigger and better the improvement from using a DBA.
The bass you get from a DBA is so good you will probably be set for life. Depending on how you decide to go it could run you $2k to maybe close to $4k. If you're willing to do a little DIY then Audiokinesis has a kit you screw and glue together that is hard to beat. Or you could buy four used subs, of just about any type. It really doesn't matter, as you will see looking at my system there are 2 ported subs, 2 sealed, and one powered. So don't worry about which sub to buy. Stay focused on getting four and you will be fine.
If you do the tweaks your soundstage will open up, your speakers will disappear, clarity of detail and dynamics will massively improve. If you go the DBA route then soundstage detail won't improve as much, but you will feel a much greater sense of envelopment, of no longer being in your room but being in the recording hall instead. You can get more bass with one or two subs. With four you get this awesome feeling of envelopment.
So there you go. Take your pick.