I like Mark II reasons why some don't like it.
It gives me just enough gain and break up, like a Fender w/ a pedal. (For me) Switching between the two channel lead / rhythm without too much trade-off or compromises.
Mark III, I would like a setting for lead, but switch to rhythm channel, I would have to dial the tone controls differently. Just could not get the amp to where I like both channels. And adding a second mode to the rhythm channel got even more confusing. Don't get me wrong, its a great amp its just that I like the simplicity of the Mark II.
The Mark IV is sensational amp, versatility in such a small package.
Personally if I need that ultra high gain, I would go for a Dual Rectifier.