I read a post on harmony central where a guy was having my exact problem and solved it with a 4 x 10.
Hey! I'm that guy! 8)
Yes, I found that the 4x10 made a big difference for me. Especially in the beginning when i first got the amp. I'd started with a 2x12 and ch2 just wasn't working for me.
The tighter low end response of the 4x10 really helped me clean up the flab in ch2 esp. with bridge pickups.
That being said though, if what you're looking for is a gainier copy of ch1, then the mud is kinda unavoidable.
Mesa suggest turning the bass down as you turn up the gain, because the gain circuit adds bass, this is very true and something worth noting.
The big thing i've found is that the 'mud' is actually still there in ch1 on clean settings. It's a HUGE part of what makes the clean sound so big and beautiful. I'm mainly talking about chords here, lead lines benefit, but it's not so much a problem.
Unfortunately I think adding gain to this sound takes the highs off and exaggerates the bottom end. you can turn down the bass control, but what you really need to do is fiddle with the other controls too, all of them unfortunately. The sound is in there, but it takes a fair bit of work.
The biggest secret i've found to getting CH2 to cut through (esp. for chords) is using the thick setting to get a more nasally presence to the sound.
In fact in order to get an gainier copy of my ch1 sound, i've found that i need to pretty much stop looking at the amp and dial it in with my ears. it's easy to just copy your settings from on ch. to another, but unfrotunately the gain structure of the amp just doesn't work like that...
For me now, i'm EXTREMELY happy with the amp, but only after 6months of working with it hard both live and in the studio.
I really feel that i could properly dial in a 2x12 now if i wanted to (i haven't tried though so don't quote me on that!) for me, getting the 4x10 really helped me like the amp from the start, but it was using it for 6months that made me love it.
I hope this helps a little.