I felt like I got close with Mark I, bass 0, mid 9, treble 3 and then used the GEQ with bass sliders way down low and treble up pretty high sorta like a slant / - you can check the settings thread for a pic. I also got something out of Crunch that was kinda getting in the neighborhood by really cranking the mids to like 3 oclock. I was comparing this to a TSL which is probably considered more of a modern Marshall versus a plexi or whatever. Dunno. I am sure others know more about it than I do. The problem is that the 6L6s just don't break up like the EL34s. When I switched to ELs, channel 2 really sounded great, but didn't like channel 1 after that.
Oh, and as rika said above, try 10 watt mode, it helps channel 2 break up a little easier at lower volumes.