Couple things you may want to try:
1) Keeping the gain where you want it, turn down the master volume. The mics may be overdriven if too loud. Record a "scratch" tract of sucky guitar initially with the rest of the band if needed, and go back and re-record it quietly later.
2)Locate the 57 just offcenter and 3" from one speaker-forget about the rest.
3) always record "dry" or flat with no EQ. You can always add it later, you can NEVER take it away after it's on tape.
I made some excellent recordings on Fostex equipment over the years and there is no doubt you have the tools to do it too.
