Thorens TD 166 hum

It might be the ground tag on the Shure. Remove it and see if that solves the hum problem!!
https://service.shure.com/s/article/how-to-remove-grounding-tab-on-a-shure-phono-cartridge?language=...

Thanks, will try this. But it does not explain the louder hum when I remove the headshell, or is this a normal thing?

It is normal to have hum with the headshell/cartridge removed from arm. Do you have the tonearm ground wire connected to the preamp? All tonearms require this!

And hum becoming noticeable at 11 o'clock is normal. The M97 has a rather high output. Don't go performing surgery on it (ground tab removal)! That's a bad idea!

Why is that a bad idea? This is from the Shure website!!

If hum increases when a human hand is brought near the phono cartridge headshell, it is likely that the metal shield in the cartridge is not being grounded by the connections. If a plastic headshell is used, the metal grounding clip on the cartridge terminal marked "RG" should be in place. If a metal shell is used, and the hum is steady, removing the grounding clip may help. If the hum persists, replace the clip and proceed to next step.

I have not removed the grounding clip as unless someone with a Thorens TP 60 headshell or a TP 11 tonearm has done this to solve the mid volume hum, I don't think it will solve anything. The headshell is metal, aluminum I think, but due to the way you mount the cartridge, it would be similar to a plastic headshell. I tested with a meter and the shell does not conduct. I tried the service manual suggestion of removing the tonearm ground wire from the right shield connection and making it and the chassis an independent ground wire, but no change. I removed the independent ground wire from the amplifier ground post, and still nothing changed. This has me further puzzled as I would have expected the table to produce a loud hum like many units when you don't hook up the ground wire to the amp.
When I checked for continuity between the tube or gimble of the arm and the ground wire, I got an open circuit. Only when I touched the base of the arm did I get continuity.

Ok, I redid some of my continuity tests on the tone arm now that I saw where the grounding screw was on the arm.
I measured from the ground tab on the Shure and the screw at the back of the arm and I get 2 ohms.. I tested from the headshell collar and the screw, same 2 ohms. I measure from the screw on the tonearm and the ground post on the HK 930 where other modified Thorens ground is attached, and I get 3 ohms.
Next test will be between arm ground and each of the cartridge wires (R, W, B, G) as well as the pin at the cartridge and the RCA outputs. More to come...
BTW, I’ve modified the original grey wire to convert to female RCA jacks at the rear, but used only a small section of the original grey wire between the tonearm connection and the rear where I soldered the RCA jacks. I did this thinking that the original grey cable jacks were faulty, but the hum remains.
Even though logic says removing the tab is not the solution, the Shure hummed ever since I mounted it on the Thorens. It had no issue on my luxman px-99 and the Shure M95HE on my technics 1300, which is the other table attached to the HK 930 is dead quiet.

I tested all the other cables and all was good. 
The last thing I did was replace the 6ft RCA cable with a short 3ft one. The 3ft cable ends fit more snug on the seemingly smaller RCA inputs on the HK930.
Success, hum is gone!
The 6ft cable ends fit loosely at the amp. I don't know if that was the issue or if it was the length.
There still is a little hum, but only at close to full volume which is mimicked by the Technics connected to phono 2.