On Saturday night, my AT800 uttered a few horrible noises (even more horrible than my normal playing) and then ceased to emit any noise whatsoever.
Yesterday was diagnosis day. The combo has an amp in a sleeve at the back, with a jack plug connecting speaker and one output. With the jack plug unplugged and a speakon cable connecting the amp to a known good cab, I got sound. Reversing the experiment and connecting the jack plug from the speaker to a known good amp, I got a little sound from the tweeter. I extracted the main driver and checked for continuity - open circuit. The driver is a Sica, custom made for GR Bass. Considering this amp has only done about 50 hours of gigging, keeping up with a medium loudness drummer, this is rather disappointing, and I'm trying to get a replacement driver from GR Bass as a warranty issue. Fortunately I have a spare amp and cab which I was intending to give a run out to anyway, so I'll be using those until I get a replacement speaker.