Uh, wat?
An 'MGL that will blow a Marshall out of the water'....maybe some, like a DSL/TSL/900/etc but a good 2203, be it an 800 vert input or a JMP, or going back further to a vintage Superlead 68-73 I would predict a big negative ghost rider.
Don't know how many vintage stock or modded Marshalls you've actually played, but it's pretty damn hard to top a healthy vintage Marshall boosted or modded. I haven't played an MGL, but I have owned some Wizards and not much out there pushes/thumps like a 100w Wiz...except an old Superlead.
I've heard the MGLs are good. But, if you're after a Marshall sound, get a Marshall. I had a Nick modded 69 SL, and another had a Nick modded clone at the amp fest in Chi a couple yrs ago. The clone was cool and killer in its own right...until we compared mine which had OG transformers vs the clone which had the really good Merren clones....the difference was HUGE. The OG made the clone sound flat in comparison.
I'd give you the benefit of the doubt on MGLs vs 'some' Marshalls, but all?
Nah, no chance. Unless he's stocking his amps with real deal 60s/70s transformers. Those just can't be replicated.