FWIW:
I turned a BQS control into an outboard preamp (Dimento's preamp experiment), used it with a bunch of different pups and never had a problem. In fact I used it with active EMGs and it worked fine. Can't remember whether I ran other make actives through it or not. Don't use preamps any more so they're all history.
To me the variable mid was a pain cause I couldn't find a tone and if I did I couldn't get back to it. To make it worse, the EMG's don't have center detents on the mid control. I wired an OBP-3 with a variable mid and it was the same way, but at least the pot had detents (cause it came from a Bart harness). I'll take a mid with just boost/cut or boost/cut with a switch over a variable mid any day.
I had a BTC in a bass with EMG PJ's and it was a pretty decent preamp. Also liked the NTBT. Didn't like the OBP-1. I'd prefer control over mids but I'd take a good 2 band over a 3 if the 3 had a variable mid that had to stay (like the EMG, for the OBP-3 it's an option).