Thank you for the advice Gonk. I currently do have the amp hooked up through the record out. Thus, leaving the volume control to the amp as you pointed out. As well, the main pre-amp for the input selections. Digital sources do not work through the record out.
It sounds very nice though spinning vinyl using the AKG 701 cans through the tube amp. A huge improvement over the 990's headphone out, but at a considerable cost that I would not have done if I was not upgrading this headphone amp. The 990's headphone output is not bad, just not as good as a much more expensive tube amp.