What swordfish said but I'll add my experiences.
I've run AT-853's with a 4.7k mod into a UBB and have had some bad luck of late with levels. As explained to me, the mod will drop levels on the mics by around -10 to -12dB which explains for "lower levels." As Chris originated this mod, and I believe his mics have the same built into them, this could explain your low levels. As mentioned, you can run the mics > battery box > mic in (plug in power ON) and this will solve your problems. Some shows (especially in smaller dinner style venues or other such small rooms) will never be loud enough (i.e., if you don't need to wear earplugs, it's almost guaranteed be "too quiet" for line in purposes). In these situations, a preamp might help but if you're trying to run super low-pro, then the mic in solution is the only real option available.
Hope this helps.