You can mod the Marshall cab.
I suggest swapping the G12T 75s for a pair of G12k 100s, G12H 75s, or V30s. The X pattern is not something I favour. I prefer to pair like speakers on either side of a cab so that it sounds more stereo and 3D. I think it is the G12T 75s that are farting out and contributing mud. You need tight speakers because with metal tones, speaker distortion is undesirable. If you can find who sells OEM mesa parts, you can order 2 OEM Mesa V30s. They are similar to the chinese v30s but they have a smoother top end and are less fizzy. I have a Mesa v30 and a Chinese v30 in a 2 x 12 and they sound great together.
To mod the Marshall, if you have a mediocum of wordworking ability, you can put a ported baffle in the centre of the cab to break up standing waves. It will tighten up the lows and the cab overall.
Basically, the ported baffle runs from the speaker baffle to the back panel around the middle of the cab and it has five holes cut in it. Look up the Mills Acoustic cab interior pictures to get an idea of how it looks. They sadly went out of business, but I built an O.S. 2 x 12 with one of these baffles and it is super clear and tight sounding.
https://www.gearslutz.com/board/attachments/so-many-guitars-so-little-time/441174d1420584118-damping-speaker-cabs-insideview.jpgThe back panel will then fasten onto this centre baffle which will reinforce and tighten the cab. The ported baffle will break up standing waves in the cab which will reduce the mud.
Additionally, you can seal all the wood joints inside the cab with silicon which will also tighten up the sound. Marshall cabs sound good so I think the speaker swap and tweaks should make it work.
RE: Amp. IF you want to look at other amps to address mud, the ReVv Generator 100 & 120 are incredibly clear and articulate amps with great note separation. Would definitely help out a lot in the low mid department.
Another option RE: cabs is to go with two 2 x 12s. 2 x 12s tend to be tighter because of the reduced acoustic space and they are lighter and stackable which is great for transport. If you wanted to run 4 2 x 12s for a fullstack, you can use series boxes so that the impedance correctly matches the output on the amp.