Part of why I like Mark's harp playing so much is his tone, Dave; he doesn't (or didn't in '80-'84 when they shared a couple of bills with acts* I worked for) have that harshness. Part of that may be that he was running his Shure Dispatcher "Green Bullet" mic into a daisy-chained series of (IIRC) tweed Twin, brownface Twin, and blackface Twin Reverb. But he also has a smooth soulfulness that sets him apart, as well.
*The guys in one of said acts - Big Twist & The Mellow Fellows, who I posted a little while ago - invariably referred to that other guy as "That non-playin', non-singin' mother-F***-er George Thorogood!"
Peter (who does like a good harp - but none as much as Mark's)