Can you further troubleshoot by touching the headshell, tonearm itself, wires to styles, and all over to determine how/if this affects the hum?
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I assume you are using an SDS power conditioner. The old VPI PLC generated gobs of hum. The only way to remediate it was to place it physically far away from the turntable, preamps, and phono cables.
Could your rig be close to a high voltage power line? Maybe in the wall? If you are replacing a turntable and your old turntable had no hum, then this is probably not an issue.
I don't know the ET II or the SA500 well enough to have a coherent thought. Good luck.
I had the same problem with my HW-19 Mk IV upgraded with all TNT parts with a Walker speed control. Looked for two weeks before I realized my SAMA was touching the base of the turntable. Once moved and put on a mousepad, VOILA - no hum or vibration. Make sure nothing is touching your turntable base.
The first thing you want to do is attempt to isolate it rather than keep guessing. Start with the preamp, disconnect the cables, if no hum, try the cables, try different cables, try unplugging turntable, in other words, make substitutions to try to isolate when and where the circumstance is that makes the hum. It sounds from what you have said so far that it could be anything from the preamp to the cartridge or anything in between.
A few ideas to try would be try mm inputs, different phono cables, unplugging leads from cartridge, unplugging nearby equipment.
Another area to look into is the cartridge clips and their connection to the tonearm wires. Reverse the ground wires at the cartridge to determine if a wire or a cartridge channel is bad.
You hadn't mentioned if the hum is from both channels or just one. Maybe you could more fully describe what is and isn't happening.
The old "Grado Hum" is typically from proximity to a running motor and is generally not loud. Since you have a loud hum even with the motor off, it would be unlikely that that is the problem, although you shouldn't rule it out completely.
I have a TNT 3.5 w/JMV 12.5 arm. SDS.
I have had problems in the past with loud hum on the phono stage. Here are some things to consider.Loud hum usually is a grounding problem.I am assuming you only experience the loud hum with the phono stage.If the A/B tests of different cables,amplifier,placement of SDS/motor, cartridges do not eliminate hum,you should replace tonearm wire. This wire is easily damaged at the time of installation. A faulty grounding of the tonearm wire produces one of the loudest hums. Hope your lucky enough to find the problem quickly, before it drives you crazy.