Thanks for the tips. Here's what I did (and what happened):
rec 1: mic-in level 67 sp-cmc-8 w/ at853 adaptor and at853 (omni) > battery box made by friend 72khz bass rolloff > dr2d [24/96]
rec 2: line-in level 97 sp-cmc-8 (4.7k) w/ at853 adaptor and at853 (subcards) > SP bbox no rolloff > dr2d [24/96]
rec 3: sp-cmc-4u (4.7k) (cards) > SP bbox 69khz rolloff > r09hr (level 42) [24/96]
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I tested all 3 9-volt batteries before (and after) the gig and they are all strong, so the issue below almost certainly isn't from a dying battery.
Recordings 2 and 3 came out excellent. My levels were a bit conservative (peaks around -20) but I'd rather run a tad conservative than too hot and be screwed…especially running 3 stealth where it's risky to check in the field.
Recording 1, a different story. Horrifically distorted. I'm not sure if it's (1) the home-made battery box, or (2) the lack of 4.7k mods at a loud show. But the beauty of running 3 recordings is if one is a disaster, it's all ok. This was my wildcard, and given the annoying chatter around me I think the omni's would've been the worst sounding regardless.
I see others are having issues w/ sp-cmc-4/8's (w/o the 4.7k mod) w/ the DR-2d so that might be the issue … it was a pretty loud show.
This was a bass-heavy show so now I'm 100% confident running the 4.7k mods w/o bass rolloff.