I must confess, I’m assuming the HD6XX is the same (sonically) as the HD650 based on what I’ve read here and other places and what is said in YouTube reviews of the HD6XX, as well as that text on the box.
As you, I don’t hear any “veil” on the HD6XX. I doubt there has ever been one (with either HD6XX nor HD650). Not with fresh/good pads anyway. With worn pads however, it loses a lot of its sparkle and “toe-tapping” properties. Then I can see it being called “veiled”, “slow” and whatnot. One of my HD6XX actually had bad pads when delivered (strangely enough) and it sounded just like that; lacking sparkle, generally just sounding “wrong”, and also “veiled” compared to the HD600 (and my pair of HD6XX with good pads). When I changed the bad pads with new ones ordered directly from Sennheiser, all that was gone and they sounded great again.
“Un-veiled” if you will
I have also done my own comparison of the HD600 and HD6XX earlier as I too own them both. What I found is that they do sound very much alike in sound quality and signature/tuning. The HD6XX has maybe a bit more bass and the HD600 a tad more treble. But the most important difference is in presentation. The HD600 puts me right on stage, together up close with the singer and musicians, letting me hear everything up close, while the HD6XX puts me a few rows back from that stage. But even so, or maybe because of it, the soundscape feels like it has more room and space with the HD6XX. When it comes to detail, I still hear everything with the HD6XX, nothing is missing, if anything, it is actually is a bit more resolving and has a bit more separation between instruments. Of course, this is my experience with my equipment and my ears, but
It is just about on par with what I’ve read in comparisons/reviews of the HD600 vs. HD650, also moving me towards the conclusion that the HD6XX is (sonically) equivalent to the HD650.