Your best choice is to feed the direct box from the mono output on the back. You could feed two direct boxes from the L&R outputs and take two channels on the board, but the soundman will just think you're crazy.
The F2-B isn't optimal for DI. The only outputs are post gain, so when you turn up on your F2-B, you'll disturb the mix. Also, if you're running at a low volume setting on the F2-B, you'll have reduced signal-to-noise ratio because the output from the preamp will be relatively low. If you have an input gain control on your stage power amp, you want to set your preamp output level relatively high (12:00-2:00) and control your amp's volume with the power amp gain controls.
If you are running effects, you'll want to intercept the signal right before the preamp. In most cases, this will be right after the effects unit since there's no effects loop on the F2-B either. In a perfect world, you'd have a line-level effects loop to feed your boxes with optimal signal levels (and the DI as well), then come back into the preamp and have a gain control that controls the final output level but doesn't affect the line-level sends.
