I do share your misgivings about metal tweeters with classical music. They always make grand piano sound like a metallic harpsichord to me.
IDK if fitting a tweeter zobel of a about 7.5R and 0.68uF helps. A bit, IIRC. But I don't like spitty soft domes either. TBH, a ring radiator would be fun to try. Vifa XT25, SS R2604, SB Acoustics? Something to do with pinning the centre of the flapping soft dome that works.
Anyway, I had a look at the MTM PL14WJ design.
http://www.troelsgravesen.dk/VifaPL14WJ-R2604.htm. There's scarcely a cigarette paper between the Vifa PL14WJ and the SEAS CA15RLY so you could use most of the ideas:
SEAS 5INCH.
20L is about right. I think these might be great little speakers. MTM's gain 6dB level without extra distortion, so go much louder and sound authoritatative. IMO, combing just sounds like off axis rolloff. I had a good listen to something like this recently and liked them a lot.
I don't think you can go far wrong here.