I live in MN fly me out and we can work on it together.
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It sounds like there may be more than one problem. Best bet is to disconnect and unplug everything and then hook one item at a time back up and turn on the power and listen. So start by hooking the ST-70 amp to the left speaker and turn it on an listen for the hum. Next check the right speaker. If no hum, then turn off the amps and connect the interconnect to the input on the left amp (but not on the preamp); turn on and listen. Repeat with right amp.
Now try hooking up the interconnect to the left channel preamp output and turn on and listen for the hum. Now plug in the preamp and turn on and listen for the hum. No hum? Then you can turn on the preamp and the amp and turn on and listen for the hum. Turn the volume up about half way and try all the inputs. Repeat for the right channel.
Next step starts with disconnecting all four headshell wires from the cartridge. Connect the tonearm cable to the preamp phono input. Turn on and listen. Turn on the turntable and listen. No hum? Now connect those headshell wires to the left channel outputs of the cartridge only and turn on and listen. No hum? Play an old record. Now repeat for the right channel. If you get to this point and the hum is gone, then enjoy the music.
If the hum appears during the phono cartridge step, then it might be caused by a magnetic interference from the turntable motor or one of the power amp transformers being picked up by the cartridge. Re-locating the turntable might help or changing cartridges might be the solution.
Remember, no short cuts... Take everything appart like you are moving and rebuild the system piece by piece.