looking for advice on budget subwoofer

I have a budget system (rotel ra-932 integrated amp, B&W DM601 speakers, jealous?) and am looking to improve it a bit.

If anyone has advice on a budget subwoofer, or other ways to spend $300 or so to improve the system, I'd be curious to hear.

I just use the system to listen to music (no movies) mostly rock and jazz.

It is in a 12 x 12 room.

many thanks!