Fundamental difference between the two models is that the Rava is a sealed acoustic suspension design whereas the STF-2 is a ported design. From the specs it looks like the STF-2's tuned frequency is about 25 Hz, which is also about where the in-room response tails off on the Rava as well (at least this is what it measure with my Rava at home). The difference between sealed and ported cabinets is that the ported sub's response tails off very quickly after the tuned frequency, whereas the sealed sub starts to drop sooner (assuming identical drivers and amps) but tails off more gradually. In addition, the ported cabinets are more efficient and can typically go louder.
Sealed subs require more power to get up to higher SPLs, but subjectively they can sound tighter and more "musical" because of that more gradual roll off at the low end. That's why when I was shopping for subs, I specifically looked for a sealed design.
But, no matter which subwoofer you go with, I would strongly recommend that you also include a parametric equalizer with your purchase, especially if you know that you got some problematic room acoustics. Even a tight low-Q design like the Rava sounded unbearably boomy out of the box in my room. With some measuring and tuning using the parametric EQ, the sound is now very tight, extended, and well integrated with the other speakers. IMO, an equalized subwoofer will more often than not outperform an unequalized sub. That's why I think that these subwoofer comparisons are often way off-base, because they have no provisions for addressing the room acoustics.
My before and after frequency response is linked below.http://hometown.aol.com/sfwooch/myhomepage/subtest.gif