I got these drivers from aliexpress:
top: 40mm "carbon driver", these must be the same as cellulose drivers according to some members here.I got these around 2 years ago
Middle:50mm Berylium 300 ohm driver, these are 50 Eur per pair, plastic housing.
Bottom: 50mm "K100"driver, 32 ohm. 58 eur per pair, the housing seems like blue anodized aluminum.
I haven't put any of these in any enclosure to try yet, except the 40mm. Brief listening: very powerfull bass.
Just judging from appearance, between the 50mm drivers, the "k100" looks better engineered. Aluminium housing would ensure there will be less resonance and the housing is more stiff. And without judging that the k100 driver is not entirely coated in metal along the surface, looks like the coil diameter is larger. In theory it would make more stiff surface and lower surface distortion at higher frequency. The Beryllium coated driver looks exactly like the titanium coated driver they have all over aliexpress few years ago, which I also got. Therefore I suspect the manufacturer just do the Be version just because they can, but with little engineering thought put in. The K100 looks like more efforts are put in.
I'm designing and making 3d files for the enclosure and will make the 3D files for sale. I have the basic design after years playing with the HD800 drivers and hopefully it can help people interested in diy and playing with drivers. THey should fit all 50mm drivers and 40mm with adapters. You should be able to tune the sound on the front side and back side to, so the headphones can be fitted with any drivers and any compatible sized pads.