[sssboa]

  I'm in the same boat.
I know the STX can drive the headphones fine,but I'm pretty sure there's some untapped potential left.
Any advice?

No untapped potential mate, DT990 600 ohm don't sound well with valves, too warm. I tried everything, A1, tubes. They sound their best with Xonar Essence, they are not underpowered at all. Actually they sounded underpowered with A1 comparing to Xonar. With tubes I tried so many valves but DT880 600 ohm sound just much better with tubes. Strangely the mobile amp Headstage Arrow 3G (the old model) powers DT990 600 ohm very well, better than A1, also you can choose output impedance. DT990 have great response with virtual surround systems.