Regarding low and high level inputs, the few guys that have made these and tested them extensively all say the same thing: that they sound better running high level inputs paralleled off the main amp instead of low level (RCA, XLR) inputs. It's not just a gain issue, the claim is that they blend better in that configuration.

I guess that kind of makes sense since you're taking the sub inputs right off the main outputs. Whatever characteristics that are inherant in the main are translated to the sub amp.