I chose to go with PEN with vapor deposed Be for a couple reason's: 1. The same as you state above, there's a sharp nature to pure Be and many metal drivers that didn't totally gel with my aural taste. I found pure Be material for actually a lesser up front cost on R/D and material. But it just didn't sound the way I imagined for a Be ZMF. 2. The combination of PEN (Polyethylene Naphthalate) and Be brought a stiffness between that of the Atticus driver and the auteur driver where it's nodal enough to sound very punchy and stiff enough to "surround" you with somewhat planar like sound in a dynamic. They are quite different. I didn't want to come out with a headphone that replaced another. Not my intention in building these. Remember I have to expand my personal collection of headphones too haha!