Re: What's the best & easiest way to thicken up O/D channel?
One difference between solid state Marshalls and tube ones are the width of the notes. You could try correcting it a bunch of ways, but maybe it's time to swap it for a a DSL 50, or another tube amp. Just a thought. Or at least run it into a good Marshall 4-12.
For fattening a tube amp, I'd say to put a GT 12AX7M in the first position and JJ's for everything else.
For a fat clean boost/Overdrive pedal, I'd recommend the Banzai Coldfusion.