Why do you need two headphones but with a single volume control? Are they matching in term of sensitivity/impedance ? I believe HD6XX is harder to drive than Focal Elex, it should be HD 6XX to 2.5mm jack.
I think the headphone output 6.35mm and 2.5mm are shared power, try to unplug one of headphones, do you hear loudness?
I was doing it for AB testing, and yes it makes more sense for the 6XX to use the balanced 2.5mm jack, good point. I cannot hear any change in loudness when I unplug one though.