I'm certainly no expert as I have only had my rig in working order for a couple weeks now, but I'll throw in my $.02 anyway. I usually have the crossover set to a more moderate level because I have a 212/210 setup, and the 212 can certainly handle some of the mids along with the lows. On the F-1X, I usually have bass set at 4-5, mid at 5-6, and treble 3-6 depending on the sound I am going for and the environment. I rarely adjust the HF, but will occasionally feel the need for a little boost or cut depending on the room and the way the sound is carrying. I found that when running my home stereo through the F-1X and a pair of small Alembic cabs which I recently acquired, the HF needed a little boost to brighten up the sound. I can get all the brightness I need on bass by opening up the filter all the way; and then there's always the 210 cab's attenuable horn for more tweaking...
Adam