I'll be able to comment further when the HD7 DJ arrives here (and it's on its way). I haven't heard the HD7 since CES, but I remember it as more bass emphasized than the HD8. Like it always is during CES, it was loud in the Convention Center, so, again, I'll wait until they're here side by side to say for sure.
They seem to be different, and I can't get the MOMENTUM's plug to fit the HD8, nor can I get the HD8's plug to fit the MOMENTUM. Other than the obvious difference in plug length (the HD8's plug is much longer), the lock slot is wider on the HD8's plug (it's a similarly configured right-angle lock slot with the little detent tip, but a substantially wider slot).