" All four models have the same tweeter; it’s made of Boron, not Beryllium. "
Clarify mac742's statement
The tweeters are some kind of aluminum/magesium mix with Poron used for its suspension.
From a materials site:
PORON®
Urethane Foam Material
What is PORON®?
PORON® is a microcellular urethane foam with extremely low compression set. PORON materials are high density, flexible cellular products and are manufactured by continuously casting and curing mechanically frothed urethane intermediates to the desired thickness. By controlling chemistry and density, the properties of PORON®materials can be widely varied
I too find the Aria series quite nice. Almost bought a pair of 948's awhile back. Went with an older model ML stat instead.