Hi, Where is the source of the noise you hear exactly, and when? Unfortunately many things can cause a problem you have, even positions of the cables.
I know one guy who complained about a constant noise from the Sub when the system is turned off just on the control pod. He checked the two main 10000uF filter capacitors and one of them had the top cracked up. It means for the cap the game is over. On other forums i found the Su'scon caps have a tendency to fail without external signs, so no way to be sure anything without proper equipment, or replacement.
I have to agree with the repairmens in general about the the strength of the soldering. In most situations the the legs of the components rather break out from the body instead pulling out from the PCB, but it is not true about the Z5500. I bought 12 Amplifier board for repair and upgrade recently, and thanks to the poor packaging many of the big caps completely break off form the boards in one piece during transportation. Just for fun i have done the same with few others to see, and happened so easy. Cheap stuff......
The Sub have two TDA amps, if one of them failing it make sense the repair men want to change them both, but the other channels have one each, so to change all 7pcs is clearly not necessary. Providing schematics with products for repair purpose went out of fashion in the age of throw away customers. I can imagine easier to replace all the TDA Amps instead wasting time with reverse engineering and get the clear picture how it works and pick the wrong one.
You can find many videos on YouTube what other people used, to replace the speaker of the Sub. The original is: 188 watts RMS 8 ohms 100Hz 10% THD. Using it in the car is technically possible with a 220V inverter, still i doubt it is wise because not designed to use up the available power on the most effective way, like a car amplifier. A simple modification to make it run from 12V is unlikely.