Honesly if you can't get the THX amp then The Atom would be a better buy with your headphones. Both the DT1990 pro and DT770 pro are 250 ohm (assuming you have the original) and neither of them have an option to go balanced. I think they will be a little underpowered by the SH-8.
Also if you don't already have a balanced cable for your HD600, a good quality cable will cost you between $70-$100 so you might as well just go for the $100 Atom amp. It has enough power to drive all you headphones with its single ended output.