one my favorite topics, so i will chime in, even though i have never used an ad20...
adding to what dwonk said - i run the heb's (dpa4060) with the core-sound batt box. it is stock, and i cant remember if the box applies a gain, it might, but i have those cables terminated with rca males, and run directly into the oade line in mod (if you want to run line in with the sbm1, you MUST use the dual RCA jack). but i dont run a seperate pre (and is likely a reason why i dont find the 4060s too sensitive)...
also - the freq switch can be knocked, as can the mic / line in switch. i popped them both off after setting them to 48kHz and line in, respectively. you can still switch them with a tootpick if you need to (popping the plastic cover off with a small screwdriver doesnt hurt the unit), but getting rid of them gurantees you wont switch them...
oh, ad a stock line-in can brckwall, too - trust me.
dont those vu meters people are using run from a headphone jack..? couldnt you plug one into the sbm1 and have a detailed meter remote (at least as long as the cord)..?
as for darkness (sound), dont know about that. i guess it comes down to synergy between your gear. my mics are about as transparent as you can find. so running it through a warm mod sbm1 is a nice compliment. but if you run mics that are heavy on the low end to begin with, a warm mod pre/a>d is not the way to go - you are looking for more of the transparent or presence mods...
but the at's arent that muddy, are they..? what bugs you most about your sound atm..? if you think it is too bright, the sbm1 might be what you are looking for...
too bad they are not made anymore