I use Velodyne DD-15 with my RM25si. The digital equalizer of the sub is useful in a small room like mine(16X11X9). Joseph claims a flat response down to 30Hz for the RM25si but I measure a sharp rolloff below 40Hz. That said, a lot of music doesn't need the sub. There isn't much below 40 in jazz or rock; there's not a noticeable difference for most music if I turn the sub off. I can't comment on your connectivity issues.
What subwoofer for Joseph Audio RM25si
What subwoofer will fill out the sound from my Joseph Audio RM25si speakers (currently driven from an Anthem MCA-20 amp)? In addition to the usual considerations about sound quality, I have a couple of connectivity needs to meet: I want the subwoofer to have loop output with RCA connections, and to be able to turn it on and off with a 12 volt signal. (These specifications are so that I can continue to use the second set of outputs from my Aloia pre-amp into amplifiers that drive speakers in other parts of the house.) Suggestions welcome. Thanks.