Thanks for the reply, andershoeg.
Agreed on the necessity to crank the pre's; that's normal. I usually try to have the volume on the F-50 set at between 9-10 o'clock (no loop attenuation) when recording. Bone-crushingly loud (I luckily live in a house and have isolation headphones), so I can keep the pre's gain level down.
I wonder how much is environment for me. Ever since I moved from a larger, carpeted basement to a smaller, tiled-floor recording space in my new house, my recordings have gone to crap. I wonder if the culprit might be phase cancellation from all the hard-surface reflections, though I'm not sure how much this would affect a single SM-57 located about 1" from the speaker grille. I might have to try a different room (can't afford room treatments at the moment), though none have carpeting.
I also moved from a Mesa horizontal 2x12 to a Mesa Standard 4x12 at the same time, which complicates things. However I would have thought that increases the level of bottom-end, not decreases it!
What speakers are you micing, BTW?