I suspect something do do with the DC on output protect circuit, P_GND,
68K +VB discharge resistor to floating ground. Amp used by buskers , in
for a blown amp (repairable) , but in passing mentioned this. Sometimes
first thing in the morning would fail to start, power LED flashes
(manual says DC on output detected), until you touch the matalwork of
the amp. This Stagepas and also their earler 300 did the same thing.
They will try a grounding metal plate under the inverter, it has an
earthing butterfly grounding point for this, to see if that changes the
situation, with ground connection through the IEC lead to the amp. After
the intial problem it will power down and up through the day no problem
.. Also in passing if they play unplugged then only about 20GBP, plugged
up and 300 GBP, annual income about 150,000 GBP