If you really wanted to get wild, you could run an SF-2 in stereo, into an F-2B (also stereo) into two separate power amps (or a stereo power amp).
Or if the bass is stereo, run it into the SF-2 in stereo, then use the F-2B to sum the signals to mono...
If the bass is mono, though, it might make most sense to go SF-2 --> F-1X
I haven't used the SF-2 as a preamp either, only as a tone-shaping device. There are a couple of threads on here about using it as a preamp though - would be worth checking those. If running in mono, there are basically three volume controls - the direct signal volume, and then volumes for both filters. It's important to note that the controls work differently depending on stereo vs. mono operation.