Their utilities are totally different. The F1X is an updated, vintage design tube preamp (modeled on the Fender Dual Showman). It is very strong on warmth, clarity, tuby crunch - all the things you'd want in a tube preamp. However, despite some nice features (effect loop, controllable, internal crossover, direct out), it has only the most basic EQ (t and b boost, mid cut). That's where the SF-2 comes in for lots of us. Bottom line, the F1X is a really sweet-sounding tube preamp, but not a swiss army knife type of sound shaper like the SF2.
The SF2 alone would definitely stretch many folks' definitions of preamp, because of its incredible array of 2 setable filters (can be low, high or band pass), plus mixer to blend both filter channels with your non-filtered signal. As if this were not enough (But wait - there's more!), the SF2 also contains a nice solid state preamp circuit. As Tom said, it's brought online by plugging your instrument into the front input jack. But, being strictly solid state, the SF2 preamp is less dynamic and pleasant to my ears than a tube preamp.
Bottom line - on those rare occasions that I have to do without my F1X, the SF2 is an adequate backup preamp. However, the SF2 does its best work when just filtering and mixing a tube signal.
My X-mas wish: Combine tube preamp in the same box with the SF2. I'd marry it and we'd sail off into the sunset...
Bill
