amp wiring / common ground / mono sub out
 

I have an Alpine CDA-9812RB, with stereo sub pre-out, and AFAIK no
provision for switching to mono out. I have a bridgeable mono amp for
a sub that I'm going to fit in the rear of the car, and fortunately
the battery is under the rear seats.
I'd like to use the existing rear speaker cables for pre-out signal
plus switching.
Question is, should I just use one channel of the HU sub out, or can I
sum the grounds of the pre-outs and use three of the four speaker
cables to carry the stereo sub signal? Is this necessary (LF being
primarily mono) & does it carry a risk to the HU?
TIA,
J.