How I see now,
the Sonarworks True-FI (or the Reference, not important which one) flattening
is a very useful tool with these Sennheiser headphones
(with the exception of the RS 120-II, that one is screwed up by them).
I think now, that applying the Sonarworks flattening to the Sennheisers (6XX((650)), 58X, 600)
removes most of the veils, unecessary warmth and darkness from them.
But depending on the given track, further unveiling...