Dave,
I'm not real familiar with the Ashly, but I'll take a stab at this...
The F-1X effects loop is unity gain, so the signal level from your bass doesn't change when it hits the loop. Since there's no gain in the loop, it tends to work better with low level devices such as effects pedals. Buffering refers to the fact that the loop prevents the effects from loading the bass' signal. The signal level in the loop will be less that that at the main output. I'm hoping that the loop's send and return impedances will work OK with the SC-50.
I'd suggest lowering the input threshold on the SC-50 to get it to work better with your input level. You may need to turn your bass up a bit as well. I guess you'll know the compressor is working when the gain reduction LEDs start flashing. If you can't get the threshold low enough, I guess you'll have to go back to using it after the F-1X.
I hope this helps,
- nate.