I don't own one, but the 5150s I've tried and heard others use are definitely middy amps, to my ears.
EVH's post-Marshall/homebrew guitar sound is definitely much thicker and less edgy than his original "brown" tone, so much so that I'm still surprised to this very day that he got into it like he did. If going "browner" with the 5150 meant getting meatier and losing some of the Marshall's presence, he most certainly accomplished that. Other guys who switched to them from other amps also seem to have gotten a mid boost compared to their original tone, whether there was an Axis at the headend or not.
What's really funny is my Axis Sport has MM90s, which are so different sounding than the regular Axis humbuckers, and they sound great thru the 5150 model on my Boss GT-6 (which I previously wasn't into that much), which I hear is a spot-on imitation of the original. It also sounds like they were designed to work together, though I'd doubt they were.
(That said, on a recent Roland new products video, there's a sales rep who's also using an MM90-ed Axis into their entry level modelling/FX unit and getting a killer lead sound which sounds similar to the tone I found...funny how things work out.)