I haven't played the MarkX. However, I've said many times that the MarkUS is my favourite MTS module. I like it more than a mesa Mark V, so that's saying something. It's tone is tight, articulate and cutting, sometimes it cuts a little too much. I like to dime the gain, and boost the module and i keep the gain voicing pot, or whatever its called rolled way back. I use mine in the V-EQ red voicing with all the knobs set somewhere around noon, just like a Mesa Mark 4. Pump a little delay through the amp and its reason enough to own the module and the MTS system. I can't immagine the MarkX sounding better. However, maybe it gets closer to an actually Mark V? I really don't know. When comparing the Salvo to the Mesa, I noticed how they didn't sound the same. the module has more gain and is easier to EQ. I don't know if the JF gets closer, but the tone on the Salvo is incredible, and in my case I wouldn't bother getting the JF. That's not a dig on the Mark X, I just can't imagine it sounding anywhere close to the Salvo.
It's the only module I've ever sold and re-aquired. The first one I had, had a push/ pull pot that when pulled, bypassed the gain for a clean tone. That's what I'd like my current module to have. To get a clean tone, the gain has to be on almost nil and the volume must be wide open. It sounds good, but lacks some depth. The clean tones on the module do not rival a mesa mark V. Those tones are some of the best cleans I've heard.
*It should also note, that I like my MTS setup darker than most use a stock Density/ presence board. That affects my tone in a great way!