I used to GAS for a real Marshall back in my younger days. And I ended up owning a JCM900 50 watt half stack, which was ok. But compared to the other guitarist's 100 watt Plexi, nah. Then I switched to Mesa amps, and have been a huge fan ever since.
But I've yet to find a Marshall in the Axe that I like enough to dial it in to my tastes, since it's easier to just try other amps. I really like Mesa's take on EL34 amps however, never owned a real one, but the Triple Crest model is one of my favorites. The Freidman's are damn nice-sounding too.
I realize that's not what you asked, but I wanted to just say that with so many amps to choose from, if the one you want isn't doing it for you, there's probably another that will. One thing I keep discovering about this gear, is you get to have all sorts of stuff to try out, that you probably would've never even considered playing, let alone buying, in the real world.
Another thing I'll do is swap a different amp into a factory preset, without changing anything else, and that too opens up some nice changes to the tone, without having to fool with impedance curves, or power tube hardness, or the like. I've even gotten some cool tones from non go-to amps by swapping out the Tone Stack. I swapped into a Fender amp a Mesa tone stack and was really surprised by the tone. (But I stupidly turned off the unit before saving the preset, so I don't remember which one it was.)