Hello! Thanks for the question!
The key difference between the HyperX Velour pads and the Beyerdynamic 770 velour pads is the foam. HyperX uses a very soft memory foam, and Beyerdynamic’s foam is several times more dense and stiff.
The 770 velours use the same dense foam that’s inside the 770 leatherette pads. It’s not all that squishy and barely sinks into the sides of your head. That foam allows the 770 velours to retain nearly all of the isolation and bass response found in the leatherette 32 ohm pads. The difference in isolation is on the order of 2db.
And I mention “770 Velours” specifically, because the velour pads that Beyerdynamic makes for the 880/990 use a much softer foam. Since those models are open backed, isolation doesn’t matter as much in the pads, and so they’re more comfy.
Beyer cans need properly matched Beyer pads, in my opinion. Beyer tunes all of their drivers extensively around their own pads. I can assure you that the presentation between the 250 ohm with velours and the 32 ohm with leatherette is nigh-identical. In fact, here’s a chart that I whipped up with the always-helpful headphone.com
Tyll at Innerfidelity used to run that place, and designed their measurement methodology.
The one downside to DT770’s being so reliant on their pad foam for their sound signature is that it’s basically a requirement to buy new 770 pads every couple of years or so if you want to retain the right sound signature.
Hope that helps you out!