I had the 5275; Mosfet 100; Lead 12, Artist 3203 , Marshall 5010, Valvestate 100, JCM 900 Dual Reverb...sorry to say this but good Marshall is not happening with any of those...cut the chase and get real.
(1959 2203 2210 JTM45)x(PPIMV and or Attenuator)=ROCK
Could very well be that your 5010 has a transitional circuit from the
earlier Marshall SS Amps, and i can imagine that sounding not pleasant.
The mid 80s 5010 do a generic
ACDC Sound, nothing more or less.