...using HD800S drivers. Pics or it didn't happen: Earlier unpainted prototype What it is currently (dark gray stony look) Measurements versus stock HD800S and HD650: Stock HD800S is using these leather pads: https://www.amazon.com/Replacement-Sennheiser-HD800-HD800S-Sheepskin/dp/B07MXB8XKQ They basically shifted the treble peak from 6.5KHz to 7.5KHz. Still pretty piercing. Sorry, I couldn't find stock pads. Stock(ish) HD650 FR, Distortion, CSD... How it actually sounds like to me: Well... basically kinda like a cross between HD650 and HD600 with slightly better bass extension. It's complimentary to my (still here) HD800S. So... yeah, it's possible. But I think I'll leave it up to more creative folks to figure out the rest. This is just a proof of concept to show that it is entirely possible for a HD850-style headphone to exist. I hope Sennheiser will one day consider making this headphone themselves. Some interesting tidbits that may be relevant: 3D printer used was Elegoo Mars: https://www.amazon.com/ELEGOO-Photocuring-Printer-Off-line-Printing/dp/B07K2ZHMRF/ Filter materials came from a mixture of medical mask, Amazon stuffs and Aliexpress stuffs. No shelf-liner. The pads I'm using are HE90 pads. Yes, the real ones. You can still purchase them for roughly $180 from Sennheiser. Yep, that's a Fostex headband. I ripped it off the TH500RP. Nope, no dynamat. Overall weight (without cables): HD650: 260g HD800S: 350g REF-1 (what I'm calling this thing): 275g Now y'all know what I've been doing the past few months. P.S.: yeah, I do have a new mod for the HD800S, but... I'm not going to release any info on that. It's not as if we need another HD800 mod/tweaks.