To refresh the thread, I have restored the xover 1091 of my 1993 Tannoy Canterbury to its original condition. This series did not have an earth wire yet. So I added one because I'm curious.
It is now exactly as in this photo of a Canterbury SE below in the link. And of course a connection in the plastic part where the driver connections are has bin done with a fifth socket in the middle.. I connected simple wires from the ground of the DC chassis to the ground terminal of my Quad 34. The effect is more than a little.

I'm not a guy who uses expensive power leads or RCA cables, I don't believe in voodoo capacitors wound by moonlight either. That's why I find the effect of earthing all the more evident. Some of the "shrill" tones that I had projected onto the characteristics of the HF horn have disappeared. Strings in classical music are now much more natural. A graininess in the sound has disappeared that I only now realise what was negatively present without the earth wire. The sound image is much more detailed and deeper.

I am not a technician, so I don't know if this significantly better listening impression can be transferred to other speakers. Perhaps the DC Tannoy tweeter benefits more than other speakers from the fact that "supposedly" the large chassis acts like an antenna and captures unwanted electromagnetic frequencies. And because the HF diaphragm is connected to the whole driver, it might be more disturbed than a small independent tweeter. But this is just my speculation.
However, it is a significant effect. A word of warning, I accidentally came up very shortly with the earth wire to the amp + connections. There were a spark and a loud popping noise from the speakers. Nothing is broken so far and I cannot measure any voltage difference between the earth cable and the + connection.

https://positive-feedback.com/jeffsp...onnections.jpg