The SVS filter starts working around 20Hz (probably starts a little above that, although I don't know for sure) and begins to drop off fairly rapidly below that. The SMS-1 filter starts working around 15Hz, so it is trimming away material below that point - material that the SVS's filter would get rid of anyway.

I agree with Tom Vodhanel that the crossovers being in line with each other isn't going to be an issue here (the first crossover acts on material that will be significantly diminished or eliminated entirely by the second crossover). If you knew the slope of the SVS filter and could defeat it, you could replicate that filter with the SMS-1's subsonic filter. It's just as well to let the filter SVS picked out handle the signal and set the SMS-1's for the minimum frequency (15Hz) and a fairly steep slope (36dB or 48dB will cause it to act rapidly around that 15Hz point, meaning that it will also start acting closer to the 15Hz point).