See... I went a little bit over the edge.
I have racked up an F-2B with my DS-5R and SF-2 here in the house, and then after this recent windfall, I racked up a second DS-5R with an F-1X and power conditioner and amp. I think if gigging ever happens again (and I think it will) that's going to be my go-to rig. I have an older F-1X unit racked with a smaller amplifier that I can either use for stereo, or standalone on smaller jobs, just using my outboard DS-5 unit. Or [gasp] a non-Alembic bass. My Starfire for instance.
If there's a difference between the older and newer F-1X units, I can't hear it, but there is definitely a BIG difference between them and the F-2B. (I think mine is a fairly new unit...)
I think I agree with Keith; the F-1X is a cleaner, less tube-y sound. Admittedly, I am not sure if cleaner, or modern is the right descriptor. They are both topnotch preamps. I'm pretty happy with the dual-channel unit here in the house, but fine with taking the single with X-over and DI out to gigs.
*a lot of what I am hearing could also be the frequency response of the speaker cabinets on each combination here...