Not a lot honestly. I've had a 50 watt Marshall DSL head for about 6 years now, and it's been wonderful.
The thing is, Marshalls do the Marshall sound very well. But people do not have the space to make the Marshall come alive. Even 50 watts is way too loud to cut loose and open it up to a really nice saturated volume. To keep up with the br00tal metalz times, Marshall has been increasing the preamp gain stages. I have 4 preamp tubes in my amp, and it has way too much gain on tap. Preamp gain will always sound fizzy and harsh compared to poweramp gain, and Marshalls produce way too much volume to open up the poweramp to a nice tonal level, especially in the era of excellent PA systems and great microphones
I think that a lot of folks throw money at the problem, buying independently built amps made with period correct caps and resistors, PTP wiring, etc. But the differences are marginal at best when comparing two identical circuits (mathematically speaking) build with the types of parts used at Marshall vs. NOS stuff by indie builders. I honestly think half of the appeal is placebo effect. Players fool themselves into thinking their indie build JCM800 copy sounds sooooooo much better than a newly made JCM800 from Marshall. They're the same, any difference will come down to the speakers, microphones, PAs, and guitars used.
Why are you telling us this?