Just did an install in one of these.

The architecture is the same as Toyota JBL - fixed preamp balanced line, data line controlling the preamp functions in the amp, weird crossover points, lots of EQ.

High pass:

Bandpass midbass (6x9 in doors):

subwoofer (note higher amplitude with identical volume setting):

Rear side speakers:

So our upgrade was to take the front highpass, bandpass, and rear sub signals and sum them together into a full-range signal. We could have done this with the rear and sub if we wanted to, but most summing devices have 3 pairs of inputs, so it didn't make things any simpler, really.

We used an Audio Control LC8. The signal sense didn't work, which was expected.

Front high, midbass, sub summed together without any "normalization" applied:

We took that into Zapco DC amps and did our normalization that way. Sounded great and no hiss...