Thanks - that's good to know.  The overdrive channel on mine certainly sounds good even at civilised volumes indoors...so I think you are right, the essence of the tone is in the preamp section.

I wonder - do the ceriatone HRM amps sound best when they are driven hard (with output valve distortion) or is the 'Dumble Sound' more related to preamp distortion?

The latter would be good, since it would make the amp great for bedroom use 

Expert opinions most appreciated.

A very simple solution is to use a stereo effects unit (I use an Alesis Nanoverb).  You just use the standard insert cables (TRS to 2 x mono jack) and plug them into the right and left sides of the effects processor...

Jay