Recently picked up a couple sets of the HD8. Frankensteined one together to create a pristine version. Have to say they actually have exceeded my expectations.
I have used many closed backs from Fostex TH-X00/900, MDR series incl. 7506,7520,MDR1a, Senn HD25, Amperior, 250 Linear (600ohm), 280, 630VB, Denon D2000,600.
I really quite liked the TH-X00. But I actually prefer the HD8 over them. These are actually among my favourite closed backs I have used and may be my number one just by their tuning.
A bit of background, I was a DJ in the 90s/00s, helped put on large festivals. This headphone captures that sound, being outdoors with a massive speaker stack. The rumble and energy you get from basslines and drum machine kicks are all here.
The mid range tuning and treble is excellent, it allows you to turn it up, and only by doing this do you immerse yourself in it's sound. The technicalities on display are here. Very low distortion, and an overall a very linear natural sound, though it has its veil.
This isn't accurate in the typical sense. My Focal Utopia and Etymotic I would regard as classically accurate. However, this is accurate in terms of recreating the emotion, experience and energy of large-room club/concert dynamics. One that is well-damped. Where the space creates diffusion with a roll off in the treble (obviously depending where you are in the room).
So my Utopia have actually been set aside pretty much since I've got these. They are just sheer joy to listen to. I don't think I'd recommend them for everyone or as a primary can, but if you get the references that I described above this can is the real deal. I've also enjoyed using them for movies (5.1 spatial rendering via Atmos) and combined with a SubPac which is just bass heaven.
I'd put the driver quality in this above the HD650 - and it certainly has that modern pedigree. This is a very well designed niche headphone. If someone was looking for something with a more neutral signature from Sennheiser than the HD630VB is also excellent. Though I prefer these for what I want out of closed back to compliment my other sets.
In short these are outstanding and I think have been overlooked because of their DJ status. Easily one of my Sennheiser favourites alongside HD800, HD650, and HD25.