The F-2B is a stereo preamp while the SF-2 is a filter that can be used stereo or mono. Think of the SF-2 as super Series electronics in a box. The SF-2 has an internal switch that can set it to boost the signal by about 10 db, IIRC, so it can be used as a preamp, too. While I’ve never tried this for guitar, I would expect the adjustability of the SF-2 as a preamp to be somewhat limited compared to the F-2B.
Bill, tgo