to OP, since your running off a high output converter, start simple and check fade adjustment to be sure it set to 0 or "balanced". Second i would try a different radio with working pre outs.
This is not a power issue, same amp, same battery, same wires etc as before it started happening. also check connections on the high output converter, the amp may only be receiving signal from one channel.
and capacitors are garbage, if yor having power issues get an optima yellow top, and a high output alternator.