Sorry, haven't read all eight pages, but from what I've seen, and trying to answer your last posting:
Indeed, the amp is not "overdriven" by the Overwater. It's overdriven by that guy from South Wales.
Without going into details:
You are asking one or more of the amp's stages to deliver amplitudes (say volume) that it can't. Having done its very best, it clips (cuts) off the part of the amplitude it can't deliver. This clipping results in both a lower volume than wished for AND the production of a massively awful lot of new higher frequencies. Depending on construction and possible presence of protection circuitry, this might blow up the tweeter(s) in the cab.
If the tweeter indeed is blown up, this will normally not be harmful to the amp, but in some (rare, as far as I'm aware) instances may reduce the impedance of the cab to far below what the amp's power stage can tolerate (typically removing the impedance altogether), and _that_ would possibly ruin the output transistors (or equivalent) of the amp. This too depends on construction and possible presence of protection circuitry.
Without knowing the specifics of your gear or the specifics of that guy from South Wales: you could maybe experiment a little with the gain and master volume settings. Something as simple as that can help a lot.
YMMV, and all that.
best,
bert