SME 20/2 Tonearm Ground Wire: Where in the table should be connected?

First of all, if you have no hum or  noise problem, you may as well do nothing. The only value of grounding is to reduce or eliminate such problems. Second, ground is ground (like every other sweeping generalization, there are exceptions), so all ground wires can be theoretically connected to the same grounding point, in this case the ground lug on your preamplifier. Ideally, they are all at zero voltage. That’s really all you have to know about those ground wires. Others gave you a lot of good detail information specific to SME. 

As @lewm meant to say,  If it aint broke don't fix it. If you have already connected the tonearm's ground wire, the one at the RCA end then you are done. Connecting the other one may well form a ground loop. The extra wire was meant to ground the turntable and it sounds by your description that it is already grounded. Just wrap the wire up and tie it to the others with a tie wrap. It won't be the last loose wire in your life. 

Hi @boothroyd 

Above you wrote:
The second ground spade in the phono cable loom is simply to engage the shielding - it’s the two wires exiting the RCA rears joined together. I typically prefer this connection left hanging (not attached) unless noise is evident.
As I have the same equipment, I have just disconnected "the two wires exiting the RCA rears joined together" from the Phonostage grounding port and I think there is a slight improvement in sound.
I have 2 questions if you don't mind sharing:
Do u have and explanation on why not 'engaging the shielding' improves sound?
Should I further break the join between these 2 ground wires from the L & R RCAs?
Thanks
Vusi

@vusi_khumalo 

Do u have and explanation on why not 'engaging the shielding' improves sound?

While I don’t care to discuss in absolute terms for fear of being attacked, unshielded signal leads can often provide a larger playback image that seems more open. Such results are worthless if RF or other noise creeps in - easily remedied when the shields are attached to ground.

Should I further break the join between these 2 ground wires from the L & R RCAs?

I see no benefit from disabling or attempting an asymmetrical shielding scheme.

Much appreciated !