I've been listening to the same Hartley Reference speakers since 1980.
Yes I have changed all the drivers as I have found them (always looking).
Changed out the 24" woofers just a few years ago (they came from Masterdisk studio NYC). Hard to change what you love. Although I have been through three sets of three amps in the tri-amp arrangement.
To answer your question directly: NONE!
Regards,
barts