Just received the HE-500, an addition to my humble collection (Mad Dogs with Alpha Pads, Audio Technica M50 and Koss Portapros). The HE-500 sounds incredibly good, and I can say that it's magnitudes better than the Mad Dogs although some people say they are very comparable. The HE-500 trumps the Mad Dogs in nearly every aspect (except for comfort and isolation).
I am currently using them with the Schiit Modi/Magni so I know that there is a lot of headroom for improvement. In any case, the HE-500 are brilliant headphones, and most probably, are my end-game headphones.
How many times have we heard that? See you around in some months... You will most likely realise it's shortcomings sooner or later