There is a 7506HD ?
I know about the larger MDR-7509HD, and the MDR-7506 professional.
But never seen a 7506HD.
As for the MDR-7506 professional:
The only issue with them is in the upper bass frequencies, they shave just a bit too much of the higher bass guitar edge. I think this is done so vocalists, especially male vocalists, can more clearly hear the low end of their own voice without any other freqs muddying it up too much. After all, these are the standard for tracking vocalists.