Maybe set the balance to either full left or right, and see if it fails, then set it the other side and see if it fails,
my guesses
if never fails when only on 1 side then it is a current/heat issue with amp
if fails on both sides, still probably amp or amp wiring (or maybe a sub, are the dual voice coils used as l/r in each one?)
if fails only on one side, that at least narrows it down to ½ the wiring or speakers.