If I was doing this, in the absence of any other information at all, the first thing I would do is connect the sub to its console, and then cut open the cable and hook up a logic analyser or even just use a CRO to see what is happening on each of the 15 lines, and try and identify:

(a) Whether a line is even being used

(b) If it is simply just power (3v3, 5 or even 12V) or ground.  These can then also be confirmed / differentiated from signal lines using a multimeter to trace back to the relevant traces on the circuit boards.

(c) Attempt to guess what the protocol might be on each signal line

I would start with a CRO on each line to try and identify the voltages first, so as to avoid potentially frying something else.

The last thing I would do is just connect directly to a Raspberry Pi and hope for the best.

Good tools for investigating unknown protocols include the Salae logic analyser (or clones), ant the OpenOCD software, and devices such as the Bus Pirate.

Given this is audio gear it is not out of the question that only a couple of pins are actually used, possibly with the I2S protocol, or something else entirely.