The way the wiring is done, is the sub input signal independent of the headunit's bass level control? Obviously some way to cross-over the lower frequencies to just the sub would free up the existing speakers to better reproduce the mid frequencies. Usually this means the sub signal must be "preamp", but BMW audio is known to be a bit quirky...so how did you guys get it to work in this particular case?

Also, how long is this sale expected to last?