Thanks, Dave.
You are correct in assuming that I have no effects between the bass and the F-1X. I have three basses (one Alembic), and none of them have a very hot output - the problem definitely came from the output of the F-1X, or the input of the SF-2.
I had put the whole thing out of my mind yesterday, but I came back to it today, and just started over - something very curious happened: I plugged the SF-2 into the full-range output of the F-1X, and there was no clipping.
I can now run the F-1X volume up to 5, and the SF-2 does not clip any more. I tried this a dozen times during the week, and I consistently got clipping once I passed 1 on the F-1X volume knob... but now it's gone.
I have no explanation for this - same basses, same cables, same player, same gear all-around, but the problem is gone.
Any ideas on why that might have happened?
I'm now running: bass > F-1X > SF-2 > compressor > power amp ...and it's working fine.
I am tempted to put the SF-2 into the F-1X loop again to see if that problem has vanished, too, but since I prefer the SF-2 after the F-1X, and I don't want to screw it all up again, I think I'll leave it as it is. This is the set-up I was gong for all along.
If the problem rears its head again, I will just swap the compressor and the SF-2: bass > F-1X > compressor > SF-2 > power amp
...but I'm hoping that this current configuration will continue to be problem-free.
Thanks to everyone for all the input and suggestions - it's good to know that we have a community of Alembic brethren out there who will help when problems arise!