I think the problem you're having is due to using the direct out instead of the back panel preamp outs!
There are a lot of different signal levels in the amplifier chain, and you need to feed the next guy in the chain with the proper level for the best results. The lowest level is the instrument level, then line level which your F1-X produces at the effects and direct out. The next highest level is preamp out which is produced at the rear panel jacks, then finally speaker levels, at the output of the power amp.
The max preamp level is probably around 4x the line level outputs (something like 3VAC RMS at full tilt, vs slightly less than 1VAC for a very hot line level output). The lower signal level from the direct out is why you can't drive your power amp to full output. Switch to the back panel output and you're probably going to have to deal with too much of a good thing (e.g. output).
Did you use the direct out because of the XLR connector? The manufacturer provides that for sound reinforcement applications where there's a long snake run from the house mixer to the amp stacks near the speakers. You want to have a balanced connection there to reduce hum. In your case, you want to use the unbalanced phone plug input (in the middle of the amp's XLR connectors) to deliver the preamp out signal to your amp. If the run is less than 20' there won't be any loss of quality or noise issues. You use a regular shielded cable there, like between your instrument and the preamp.
Once you've wired up, then you want to set levels from end to end again. I'd start full up on the bass, pretty high on on the preamp gain (maybe 7-8), then tweak the power amp gain so you see the input clipping lights. You might need to drop them quite a bit. I normally would set up with my preamp lower (to keep things clean) and run the power amp a little higher so I don't need to tweak two knobs to control the max output level.
Hope this helps,
David Fung
