I would also suggest the ZOTL and some nice NOS tubes for a grand, or Earmax Pro if you only use HD600s and never plan to change.
I have owned the Earmax Pro, the previous version of the MOH and the ZOTL. My enjoyment with the ZOTL is significantly greater than with the MOH. MOH is awesome, so that says a lot for the ZOTL. Only thing ZOTL does not have that MOH has is X-feed. Despite the lack of X-feed, I still prefer the ZOTL. Nevertheless, Earmax Pro with NOS was better than ZOTL with the HD600. Choose carefully.
markl