Kinda like photography, where it is sharp and smooth because it’s clear and clean, free of grain and approximations?
I know that bug, it bit me too. HD 660S removes any doubt of a veil or slowness, but still has that musical tone that sometimes the HD 800 sounds a little too bare and analytical. I really like the balance struck by the HD 660S (YMMV, but I tried to describe it).
Yeah, Stephen from Z-Reviews can be a fun guy to hang out with, but he said they were the same in his video which got me scratching my head because I had heard them before him, and didn’t agree at all. I mean, there’s some FR and impedance similarities, but even in the Z-Reviews video, after 12 minutes, he finally tries on both back to back and notes that there is actually a difference. Problem is I think a lot of people only watched the first 3 minutes, had confirmation bias hoping they could just spend $150, and have the same headphone if they take out the foam between the HD 58X’s driver and outer grille.
I have some contacts with Sennheiser, and I asked Ronja Harste and Axel Grell... they said the foam is mostly there for aesthetics (to hide the inside) and to improve the driver’s survivability in drop tests. It’s low density foam, mostly acoustically transparent. I have a bass-light HD 58X Jubilee (model at the top), and it has the foam too... just, a smaller piece, and attached to the driver mount rather than being bigger and covering everything behind the outer grille. If it is bass lighter, and both versions of the HD 58X Jubilee have the foam, it logically isn’t causing much bass to reflect and reinforce the foundational.
Please ignore the messy background of my room:
Top: HD 58X Jubilee Pre-Release (bass light)
Middle: HD 660S
Bottom: HD 650