I have an amp like yours and i have never experienced what you have described, exept when I maxed out everything and that was just plain old understandable feedback coming from my guitar.

Right, I think I have this problem licked !!

  It seems not to be a faulty amp at all (huge sigh of relief all round !).
It's all rather complicated but......I had actually been plugging into the amp through a few true bypass pedals. They were all switched off, so I assumed they were completely out of the circuit.
  These true bypass pedals were powered by a power supply that also powers other pedals on the Wet side of my Wet/Dry rig. So I have a Wet and a Dry side to my pedalboard.
  However, leads from both the Dry and Wet sides of the pedalboard are plugged into a metal-housed hard-wired patchbay box. One of the leads plugging into this patchbay box feeds the Wet side of the rig and takes a signal from the line-out of the THD Hotplate that I use between the Ceriatone Plexi and the cab.
  Anyway, somehow (and this is well beyond my understanding) this was all creating some type of feedback loop. My guitar's signal was definately only going through true bypass pedals and a true bypass patchbay, so I assumed it was the same as plugging into the amp direct. I was clearly wrong.
  The big problem seemed to be this shared patchbay box. Within the box there are, of course, no internal connections between the "Dry" side of the rig connected to the Plexi and the "Wet" side, being fed from the Hotplate's Line-Out, but I'm certain that some sort of capacitance-based (I think that's the right word) feedback loop was being created.
  After a lot of fiddling around this morning I am now only using the patchbay box for the Wet side of my rig. None of the pedals on the Dry side are plugged into the patchbay. Nor are any of the Dry side pedals powered by any PSU which also powers the Wet side pedals.
  So the Wet and Dry power supplies are now completely separate. The Wet and Dry side also now do NOT share any patchbay box.
This seems to have completely cured all the problems for the moment.

  It's done my head in trying to work it all out. My main feeling is one of relief, as the amp is good, but I feel like an idiot.

  I must also say that Nik Azam was also doing his very best to help me sort this out. Thanks Nik.

Gonna have a lie down now. It's all been a bit much.

I had a similar problem and it turned out to be the lack of shielding. I should have questioned it when I received the Mojo head cabinet and noticed that there was no shielding but...

I installed a piece of aluminum and all my problems went away.