Crest Audio Pulse 2 1100 power amps, 1999

Owner has 5 of these and 3 have gone into protect mode, all the same symptoms. All look the same but not sequential serial numbers. He's getting someone technical to look at the speakers and leads and as they are used from a diesel generator maybe a problem there. All failed in use , not abuse, not failing at switch on , and no earlier odd noises before failing. Anyway these 3 amps show clip LED on both ch, and then drop into protect. I've the schematics for them but before going down false paths ,anyone been here before. ? I've looked inside one and all healthy appearance. SMPS fires up enough to deliver about +/- 100V rails, both ch, for half a second , fans ok and relays click over but then drops into protect. No excessive mains current draw at any time. This one and the others , both channel o/p measure about 13 to 15K to ground and not tied to + or - rails so will assume PAs are ok at this stage and maybe falsely going into protect.