I'm talking about the Sony DR-Z7, 1978, which I've owned for a while: The MDR-Z7 appears to be the spiritual successor to the DR-Z7, given the design and name similarities. Both are all metal, have large metal coated drivers, are sealed and vented... etc Cheers
I'm worried about this myself as well actually. As aforementioned, the DR-Z7 uses a palladium-coated diaphragm. It is known that metal diaphragms can cause "plasticy" and "tinny" sounding frequencies, due to high odd-order distortion caused by modal breakup on the diaphragm from the metal flexing poorly. While it is very tame on the (DR)Z7 when compared to say Ultrasones (which use titanium), I can still hear audible modal breakup in the lower treble; it's rather unclean...
Different materials reflect different things, and certain treatments that you can apply to the housing further alters their acoustic properties. It's not a "one is inherently better than the other" thing, its more of using what fits your purposes thing.
Properly dampened metal can absorb a lot of frequencies, but so can properly treated wood.