Thanks for your suggestions. I've finally been able to resolve the issue(s) with the amp and have it happily up and running again.
The first issue was that the "inexpensive" rectifier from TubeDepot (https://tubedepot.com/products/solid-state-rectifier) was toast. One side shorted, the other side open circuit... It was all meant to be temporary while we were renovating ...sigh.
Knowing that AC wandering through things could fry electrolytics, I took my time and tested each and every cap for correct function. Heaps of un-soldering and re-soldering and found nothing - thankfully.
I even de-soldered the quad-cap and measured it statically and under higher-load with a testbed I rigged up. No issues found.
Next, I powered up with a Weber copper cap (the GZ34 and GZ37s were tucked safely away) and as per Bob's completely understandable point, I loaded up the amp with some of my "lesser" tubes in the front-end and some older ST-120s in the back end. It all worked and I was able to bias up everything nicely. After a few hours of function, I started rolling KT-88 back in including the ones that were in there when the power bounced.... two fried KT-88s out of the four that were in at the time - both of the right channel. They must have flashed-over but as I wasn't watching when the power pulsed, I didn't actually see it.
On the topic of a place that services guitar amps, COVID-19 messed that up and once we started returning to normal around here I couldn't find anyone who wanted to get involved with the amp. I was surprised, Sydney has 4 million people in it, surely there'd be someone who owns a business that would "have a look" but, No. So I sorted it myself. It was nice to get back to analogue with big solder joints rather than the tiny little digital pins I deal with "during the week" :-)
Lastly, Skizo, I have a WiFi (esp8266-based) power "plug" inline from the mains to the 110 step-down which is all configured into our Logitech remote so that it only brings the tranny and the amp on when I want them. But, I've re-programmed it so that the power relay is open-circuit whenever the power comes up rather than restoring the state it was in when the power was last on. Protected!
Now back in action and not enjoying the new Kanye album my son insists we should listen to...