A great mod?
I think this thread supplies a lot, and as a whole supplies an unspoken response: individuality. We each have our own wants.. dreams.. when it comes to tone. For one it might be "really close to being the ultimate Marshall," for another, "several great Marshall tones in one module," and for another "like my Bogner, SLO, Boogie, and my dream amp had an orgy, and made this child with so many flavors of tone on tap in one module." Tons of overlap, but allowing for each person to have their own unique piece.
I have wondered if the musician who desires only, say, an AC30 tone would be attracted to the MTS paradigm? Take a musician with a similar taste, but hears a Matchless and.. then hears an MTS modded module that does both. Sure, it is neither, but has flavors of both.
And, IMO, that is the essence of MTS, and by natural extension, the essence of a great mod. Versatility, uniqueness, while still being relatively affordable. The affordable part allows for experimentation, and the ease of shipping for the modules facilitates that even further.
BTW- I was going to build my own multi-channel rack preamp, but got back into MTS instead. Why? Because after seeing what all of these modders have accomplished, I realized it would be easier to mod MTS modules for myself than start from scratch.