You wouldn't really need a second amp if you're not going to use voice chat. The Mixamp fully powering game audio, will power the DT990 Pro. If you still feel you need power later on, the E9 has PLENTY of power for the DT990 Pros as they are only 250ohm.
I've ran the Mixamp to the Lyr myself. Problem with the Lyr is that it has an audible noise floor with sensitive headphones, and the tubes can hum. It's the only reason I didn't get the Lyr this time around. It was noisy with the D7000, though it was dead silent with the HE-4.
Edited by Mad Lust Envy - 1/13/12 at 7:00pm