This is probably the wrong place to ask about comparisons, since all the dedicated fans of the MD will be on this thread and all the dedicated fans of the HE-400 will be on that thread. However, here's a quick summary of what most have said between the two: The MD is a brighter, some say more neutral, headphone, whereas the HE-400 has a bit more low-end response. The MD has better imaging and better pinpoint accuracy, but the soundstage isn't as wide as the fully-open HE-400. Comfort opinions vary because the two cans are built radically different from one another in terms of earpads: the HE400 has thin velours reminiscent of Beyer or Sennhesier gear; the MD with Alpha Pads are more like the pillows of an XB500 or MDR-1R, for example.
Isolation will definitely not be any issue at all with the MD; they isolate fantastically well even with the old Dog Pads. The Alpha Pads make it even better. If you're considering your new cans for use on the train or in a public area, the Mad Dog definitely has an advantage to the hear-all, broadcast-all open design of the HE-400 (obviously).
Indeed, my Mad Dogs compare well to my dt 880s :O which are my favorite CAN, the mad dog is ofc slighty warmer with a touch more meat in the bass and a little less tightness. N yes the sound stage is intimite on the Mad dogs lol, still for a can that isolates like it does It's really worth it :D
But I don't like it for ANY electronica genre... there's not even TIGHT bass or even enough LOOSE bass xD it's kinda in the middle. Nu Jazz however [which features some NICE synth bass lines] sounds MAGICAL on the Mad Dogs, seeing as Nu Jazz combines [in my eyes] Classical Jazz an Techo [elements] The mad dogs do really well.
In addition I'm not a fan of comparing open and closed cans... as I always like open better, and I only have one TERRIBLE pair of closed cans [I suppose my Mad Dogs r my closed refrances xD]