My experience with those supposedly "correct" replacement tweeter diaphragms has been very poor. They are usually poor quality and the last ones I bought had the wrong diameter voice coil and had the lead-out wires in completely the wrong place for the dome they were supposed to be replacing. To cap it all, when I returned them for a refund, they allegedly "got lost" in the post. I would buy a complete correct replacement tweeter. The S75s are very good so definitely worth it.

Thanks for the advice, Adam. I tested the speakers again this morning with a tone generator, and both tweeters seem to rattle at different frequencies. I can also clearly tell the sound is duller on one of them. Do you think there is any chance this could all be fixed with new caps? Or does rattle generally only occur when the driver has gone bad? I'll be replacing the caps either way, as I'm sure it will help with the dull factor, but it would be nice if it fixed the issue instead of needing to drop an additional $100+ on tweeters pulled from another S75.

Just tested the ohms rating of each driver. Looks like the tweeters are both shot... Or ridiculously low ohm. Speaker 1 Tweeter - .5 Top - 5.6 Middle - 7.2 Bottom - 3.5 Speaker 2 Tweeter - .6 Top - 5.6 Middle - 7.1 Bottom - 3.4 Guess I'm spending a bit more cash.

Yes, sadly your tweeters have had it - bad luck! A low resistance reading suggests that the coil has probably overheated and individual turns of the coil wires have melted together (which would also cause the rattling you're hearing), so they won't be salvageable, sadly.