Naim seems to go well enough with the metal be-tweetered Neat speakers, by all accounts, and there was a time that (even the older) Epos speakers were a popular choice with Naim amps despite having metal tweeters.
My MS20i Pearl Edition speakers are an excellent (albeit accidental) match with my system and yet have metal tweeters.
I have heard Focal speakers (yes, metal tweeters) that even sound dull in the treble with what are otherwise considered bright or 'forward' amps.
There have recently been recommendations for partnering Naim and the new KEF Q300s. (Guess what tweeter material?)
I wouldn't worry too much about what the tweeters are made of, but instead explore the problem with a good dealer who can let you audition a range of alternative speakers (particularly if home demos are ok with them).