Julian Krause tested the SSL 2 along with its headphone output. Assuming the headphone output on the SSL 2+ is the same:
it’s output impedance is low enough to not change the frequency response of the 6XX,
- distortion is pretty bad at low impedance loads, but is good enough at 300 Ohm,
- all other parameters are not an issue.
So the amplification is good enough.
I think you would benefit the most from EQ. I suggest the preset from oratory1990: https://www.dropbox.com/s/qs4hs92tvfbwr04/Sennheiser HD650.pdf?dl=0
How exactly do you “power match” a headphone that takes literal miliwatts to reach 110dB SPL?