I use them in a music only system. Therefore, I don't care much about their performance below 30hz (I own exactly one disc - an SACD recording of the largest pipe organ in Europe - which registers meaningful info below 30hz.) In fact, I use a low cut filter to eliminate response below 25hz (was 20hz at the time of my OP) because nothing good happens down there in terms of music.
I haven't measured for deepest bass, but my guess is that - depending on the room, placement, damping setting, etc.:
A) The 12" Rythmiks will reproduce <20hz better (cleaner at any given SPL) than most 12" subs
B) If this is your goal, you can probably do a bit better at the price. The SVS tests better in this regard, though the Rythmik tests better for "speed" (group delay). I have never had the opportunity to do a side by side, so this is speculation on my part. Nonetheless, if I were buying subs for HT, I'd probably choose SVS, for music - I'll stick with Rythmik.