Grounding Speakers

My Tannoy DC8Ti speakers have a ground connector.  Can someone give me a good idea as to why I should use it?  Full disclosure - I had them grounded to my pre amp before.  I just received, assembled, and reinstalled my gear on a new Butcher Block four shelf rack.  My other rack has 3 shelf, so I’m reconfiguring the layout.

My turntable is grounded to my phone pre Amp.  My power amp also has a ground connector.  Should I ground the speakers to the pre amp, power amp, or to a dedicated earth ground?  And is there any reason to use the power amp ground?