Steve,
Right now, I'm running the SF-2 along with my F1-X. There are a lot of different ways to use it, and ways to hook it up. I can tell you that if you want to bi-amp, the F-1X is the way to go. Some folks think bi-amping isn't necessary in the age of full range cabinets. Speaking only for myself, I like being able to tweak the highs and the lows separately.
One day, I'm going to get a stereo preamp so that I can fully utilize the stereo features of my Series II and my two Rickenbacker 4001s. To me, there is nothing like being able to run different pickup signals to different channels/amps. It really opens up a whole new world of tonal possibilities.
Alan