Turntable HUM

Can you further troubleshoot by touching the headshell, tonearm itself, wires to styles, and all over to determine how/if this affects the hum?

Cartridge? Grados are known to be very prone to producing hum under the right (wrong) circumstances.

Six,

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.

Jim

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.