Just FYI, I borrowed a friend's HE-500s for a few days, and it has some nice qualities, but the soundstage on the MA-900 is better, and the drivers on the MA-900 manage to respond almost as fast as the HE-500s. The HE-500s are flatter and the MA-900 has a little bit of what is usually called "grain" that are not present on the HE-500s. The HE-500s have more micro-details.
But, all in all, I prefer the MA-900. It's much more comfortable for practical use than the heavy HE-500s, and the instrument detail of its position in space that can be heard in the MA-900, makes up for the better micro-details on the HE-500s, so the overall sound quality is comparable.