If I can just jump in with the experiences of a recording engineer still very much with training wheels on...
I asked an experienced engineer (he'd worked with some pretty big names) about recording levels, and his answer was simple: hot as possible, and don't be afraid if you get a few "overs" and redlines, but keep those to a minimum. But make sure you're using crap inputs; on my MOTU 896 workhorse I could push the levels regularly into red territory and get almost analog smoothness to the distortion, never any digital nasties.
I just hope the Firestudio I replace it with will have the same qualities.
Otherwise, I've learned a megatonne about the deeper aspects from the great, experienced members here - I'll be reading through this thread for a long while yet.
Cheers