Maybe an easy test would be to just try my receiver on his speakers next time we meet up.

He’s using a vintage HK with raspberry Pi / volumio fed Spotify.

Same lacklustre tweeter sound when we play his turntable too via the vintage HK.

Then he comes here to my place and we just jammed some Hanne Boel and Larry Carlton and he commented that my tweeters were soooooo smooth. Go figure.

M60ti Hafler9505 & JFET Pre,Axiom Transformer. M3 Marantz PM7200 Dual 606 Denon 2700 M2 Yammy RX595