I don't think you're going to find anything plexi-like inside the Legacy or any new Marshall...tube or otherwise. If it's vintage marshall you want, then you'll either have to cowboy up for a plexi or buy a copy of the amp. You could give the THD new plexi 50 a whirl, or even a univalve or bi-valve and screw with the tube configurations...but that's not going to give you the circuitry, but it might be close.
Other makers are Fargen, Mojave Ampworks, Voodoo Amps...etc...
All of those will run you some serious coin, but you've got the rectifier which isn't cheap either.
Not that you care, but I'm bored, so I'll give you my saga which contains an option you may want to consider and it's wicked cheap. I've gone round and round on amps in the last 3 years or so. Had a TSL100 head that I wasn't happy with. It wasn't PHAT enough, it was too grainy and buzzy so I shipped it off to Voodoo amps for some mods. Got it back and it was huge, but to get the most out of it, I had to crank it and that wasn't enjoyable at all for my situation and needs. Got a Nomad 45 which is great, but I missed that Marshall wallop so I got a Rivera TBR1-SL which does Marshall better than a Marshall, but that's another topic. It's basically the offspring of a JCM800 and a Bogner or Soldano.
But I still wanted to have an old marshall plexi tone, creamy distortion that you could just cut with a knife...but not willing to dump another load of cash on another amp, I took a chance and bought a Tonebone classic tube pedal. That sucker through the clean channel of my Rivera or boogie is as close to a plexi as I'll ever need. You can make it super thick, dark and chewy...it's quite remarkable.
Eric Johnson's tone is the litmus test for this sound and it's pretty darned close. If I had some old marshall basketweave cabinets with ancient greenbacks it'd be closer, but vintage 30's are only going to sound like vintage 30's. To get the old plexis and JTMs to really bark you had to dime them out, with the TB Classic you get it at much more reasonable levels. The amount of low end and tone shaping is awesome. I know this sounds like an add, but just go try one out. It'll save you a fortune if it gets you what you want. And with the extra grand you save...you can get that greenback cabinet.
So that's an option for you that might be worth exploring. For about $200 you can get pretty close to what you're after, and it's a hell of lot easier to carry around.
