I bought the speakers used after owning a set of newer DSP5k smiley faces for a while, so when I plugged them in to listen; the difference was dramatic. I thought the tweeters were completely blown. But strangely, turning up the treble tilt all the way brought them alive again. Still not quite up to the much newer smiley face tweeters, though. So it was not a subtle degradation in my case.

However, I also purchased an older DSP5KC upgraded to 96/24. I believe the tweeter is still original, however. So probably only a couple of years newer than the old DSP5k's I just bought. The highs are a hint down in comparison to the newer tweeters of my other speakers. A simple 1dB bump in treble tilt gets it right again. They are probably going down the tubes, slow but sure.

Living Room: TV-->Sonos Arc with Symfonisk rears and twin Sonos subs. Also, a set of DSP8000SE with a MiniDSP SHD for stereo listening.
Listening Room 1: G65, DSP7000SE, DSP3300, DSP3200
Roon Core: NUC7i7