It wasn't the most likely. Next to the most likely, and directly connected to it, was another stability critical component, but it was a high quality type generously rated for voltage and power. Components like this usually fail by tiny degrees, losing a small fraction of a % of their value each time. Their other failure mode is to violently explode. The first wouldn't cause a loss of stability and the second clearly hadn't happened. Acting on a hunch, I removed it and connected the tester: nothing; no connection at all. Nailed it!
I have never seen the like, before or since: the total failure of a stressed but inherently high quality, low risk component. Devices like these are often connected directly across the mains for many years continuously without failing. It must have been a manufacturing defect and could probably have blown at any time. Because I am a professional, I had a replacement of the same value, type, quality and voltage in stock so I had the amplifier back in place the same day, allowing it to be tested to everyone's satisfaction well before the big gig.
If you want Nev Ward to save the day for you or your music venue/studio, then drop him a line on the ZenWorks Facebook page