My mesa throttle box eq gets everything mesa from a Boogie to a triple rectifier. Or an acceptable facsimile of those amps for a bedroom player. Btw this is thru a Fender blues junior, so id imagine that thru a marshall it should work well.
Digitech's discontinued dirt cheap Metal Master pedal was geared to 90s Metallica tones. Does it pretty well in the right hands with a high headroom clean Fender style amp. One trick.
KEA was not an invader pickup - MJ at SD told me after looking in her log book it was a custom pickup for him with two rows of screw poles, Alnico 5 magnets. Then he did both RTL and MOP with the stock Dirty Fingers in his Gibson Explorer - there is a pic in the recent book of him playing out with that Explorer with the stock pickups after they were done with MOP. On the MOP tour switched to EMGs, never looked back. Also, some say he used TAG pickups in either the Explorer or the Jackson V. Tom Anderson told me he never sent pickups to Hetfield, but did send some to Kirk around that time, and Tom Anderson pickups might have been in their Jacksons.
Hetfield used a RAT on KEA with his modded Marshall, which was stolen. He used a tubescreamer with a stock JCM800 for RTL, and of course the Boogies with Aphex Parametric EQs in the loop for MOP and AJFA.
Cheap option... Try an EQ pedal after your gain stage, it will help quite a bit. You can really shape the voicing of your gain.
The Mesa VTwins are great little units but pricey, you'd be better off buying a boss katana amp, cheaper and does metallica tones really well, and a while lot more. Insane amps for the price, ive been saving for one myself, but... Dentist