Suggestion: Chop the problem into pieces, amplification faults versus system faults: (1) Does the unit still function computer-wise? Disconnect both pairs of speakers, reboot the device (unplug/replug) and see if your controller can “Play” to the zone, first use and afterwards.  (2) If that works, try each pair of speakers separately, same tests as you have been doing?  Culprit speaker set A or B, or does it take both?  Maybe that will give you an answer, e.g. right speaker wiring, set B, shorted causes amp into protection.  For example only...