HSU Research STF-1 Subwoofer for music?

I'm looking for a budget-price compact sub to fill in the low end on my stereo system. I currently have pair 2-way bookshelf speakers with 5.5" woofers. I doubt they go much below 50Hz, though I haven't measured.
Is the STF-1 Sub tight and articulate enough to sound good in music applications? Any other $300-$400 subs that would be better for this application?
I don't need earth-shaking bass - just something that cleanly fills a small / medium sized living room.
I own two Hsu subs. The VTF-3 Mk II is a very large one with very good output to 20 Hz and some at 16 Hz. It is used only for movies and HT, because I have large floor standing speakers in that room. I recently bought a Hsu VTF-1 which has very good output to 25 Hz but less volume than the larger ones. It is used in a medium sized living room. I have a small room with 2 channel monitors and an REL 108E to augment the bass. There are many variables. They are: the size of the room, the type of music, the roll-off frequency of your monitors (they may actuall only go to 60 or even 70 Hz if they are on stands away from walls). I would advise going for a sub that goes down to 25 Hz. The STF-1 is rated to 32 Hz. In my opinion, the Hsu subs are musical and a bargain. The sealed, non-ported subs I have heard are better at music but are much more expensive. It is worth the extra money for the VTF-1 and you can get them in real wood veneers. A used REL is a fantastic sub for the money but doesn't go quite so low, maybe 30 Hz for the 108E. However, it is sealed, small and very quick. There are many subs in the sub $400 range and Hsu subs are contenders at all price ranges. They are relatively quick and play very musical notes. Good luck!