- The MDs are
much darker, less "bite" in the treble (the 500s don't have much either but still more than the MDs) and a "closed-in" sound (vs. the 500s).
+ The MDs are
more balanced which makes details come out more easily. It's a "monitor" type of sound (in term of ambiance & details) but the sound is far from being sterile or anything often associated with "monitor".
In term of comfort, I now use the Alpha Pads with all my headphones (including the 2 Hifimans I have left, 4s & 5LEs) so that's a toss up. The 500s might be a bit heavy with the Alpha Pads (never tried, I sold the 500s just before). The Mad Dogs are still quite heavy with the Alpha Pads (~420g) which makes them similar in weight to something like the 400s with velour pads (the 400s are heavier but the weight is more balanced so, overall, it's similar).
If you're OK with a dark & laid back sound, then the MDs are excellent
If you crave for comfort, the Alpha Pads are the real deal!