The RW-12d delivers decent bass with reasonable output. I wouldn't really classify it as a "bad" subwoofer, it is probably acceptable for your purposes, but it's not really "good". It reaches down somewhat low, it has somewhat quick response, it is mostly not boomy. I will say I was fairly surprised at it's total output which exceeded my expectations.
I cannot though tell you if the STF-2 is a good upgrade or not.
HT: Samsung LN32B460 32" LCD (greyscale calibrated) | AVR: Yamaha RX-V765 | L/R: Emp E55Ti | C: Paradigm Center 1 | Rears: Sony Junk | SW: Klipsch RW-10d miniDSP | Sources: Acer Revo Ion running XBMC with mySQL library server and 4 TB HDD, PS3, Technics SL-Q2 TT | 2x Aura Pro Bass Shakers
Bookshelf: Yamaha RX-V992 | Paradigm Mini Monitors
Shelved for now: Paradigm 11 SE Deluxe