Heya,
Was just trying to see if there was something you felt like was missing from the Mad Dog that made you want to try something else. The HE400 isn't better than the Mad Dog, it's simply that some prefer it. I know others who prefer the Mad Dog to virtually everything they've touched. It's all about finding what sounds right to you. I liked the Mad Dog, I tried it with Stax pads and all, and it was a very dark, clear, warm, bassy headphone that sounded fantastic. I liked it better than the HE400 myself, simply because I liked the really dark and bassy mood it had, it made me think of a very dark Denon D5000 for example, which I also very much prefer to the HE400. The HE400 is a fantastic headphone, but I like it to be even bassier and I like isolation, so I have different preferences of course.
Definitely try the HE400, it's a great headphone.
Very best,