I'm not sure. My Thiel speakers are coax: metal dome tweeter set in an aluminum midrange (though actually the mid is a corrugated flat profile). No metallic character that I hear. It seems clear from Amir's LS50 measurements that there's enough deviations to create some sort of non-neutral character to the LS50 sound. If that (and perhaps combined with the sum total of the speaker's behaviour) is the cause of what I hear, whether that comes off as "metallic" sounding is I guess listener-dependant.
As far back as the 90's, when I got back heavily in to high end audio, I was disabused of the idea that metal drivers themselves impart a "metallic" sound. Especially if driver ringing is well suppressed, I see no reason to think the driver will have a "sound." Though, I could be wrong and might be missing something. But as you suggest, if anything is causing the impression it's likely to be some combination of frequency response/dispersion characteristics. (Again, perhaps combined with other characteristics).
Many high end tweeters (and often also woofers) are made of some metallic or otherwise specially treated compound, whether they call it diamond or berrylium or whatever else. But then again, there are highly acclaimed speakers with ("specially treated") "paper" cones in either role, too. Having listened to many, they can all sound great, and the end result is more of a function of the audio tuning rather than the material.
I know you like the power of metaphors to describe your listening experience, and if I grant that right to poets and wine tasters and art critics, hey, I grant it to you. To me however, the connotation of a "metallic sound" is some hammer on a strong metal, which isn't a pleasant sound. Give me "metallic" sound and I give you "gear return" on the spot.
Personally, if they make a better sounding tweeter or woofer out of cow manure, I could not care less (sans the odor of course :-D). Just make it work, and deliver on a well measuring speaker, and hey, let's have some fun. Manufacturers now make a big deal of the material... but the measurements tell the story. It's often about what DRIVES said woofers or tweeters, from amp end-stage to cross-over in the speaker. It's about bringing the BALANCED best out of the woofers or tweeters. If the material improves on that linearity, hey, great - but it should be totally seamless, without instilling a "material flavor" into it.
Since we are discussing this under the LS50 banner, there have been heated discussions about the LS50 generations and how relevant the audible difference is. Maybe there are people who definitely hear a 1.5dB difference somewhere in the audio spectrum (and there are a lot of measurement charts on either speaker on the internet), and I respect that superior hearing ability wherever present. But to me the funny part is when you disagree to the point of hateful anachronism despite the fact you enjoy the exact same character in sound, perhaps separated by 1dB or two at most over a very limited frequency range. But such is the nature of the audio hobby - it seems disagreement about that ultimate 1% in resolution results in religiously polarized disagreement.
Maybe I am the only one that enjoys music while not maniacially and constantly listening to that utimate 1% in ultimate resolution and SINAD and what not, rewinding tunes constantly to do blind ABC tests all day! Am I?
I like gear as much as the next person, but ultimately I like to chill and enjoy music far more, and when it is good enough - it's heavenly enough.
