The RME is a very, very good piece of equipment. And the HD650 with proper eq are very, very good headphones. I really question the need for $4k headphones and such (even though I bought the Stellia which have not arrived) after hearing these properly EQd with a top tier amp and dac behind them.
For a couple of months now I am listening to most of my headphones exclusively with EQ. I used to be a determined opponent, mainly due to past experiences with poor implementations. I have not fully utilized the EQ of the ADI-2-DAC, but am mostly relying on EqualizerAPO/Peace. Usually, I am aiming for a moderate version of the Harmann target (bit less bass, bit more treble). So not unlike your taste.
While I agree with you that the HD650 can sound very good with EQ, I can only urge you to try the HD800(s) if this is an option. In my experience one of the best headphones to eq. In combination with the ADI-2-DAC these deliver amazing low end without noticeable distortion. Also the highs are no longer fatiguing, but remain their character.