The crazy thing is this was a brand new P3 and I’ve used it without any trouble for a few months, then all of the sudden rumble from nowhere, so I moved the whole setup thinking the speaker was too close to the TT only to make the TT too far away from the Marantz and still rumble and now hum when the phono input is selected, and the TT isn’t even powered on! I have tried two different amps as well and the hum is still there, and I have a hum X on the amp power line plugged directly into the outlet in the wall.
In your current thread about power conditioners I noticed the following statement:
I have a Sony OLED TV, Marantz preamp processor, Atoll amp, cable box, nighthawk router, Xbox, Rega P3 TT, and powered 12” subwoofer with 2 stereo speakers. I have 2 Monster JP 800 surge protectors if nothing else to plug everything into.
My suggestion is that to isolate the causes of the rumble and hum problems you start by simplifying the setup as much as possible. Disconnect and unplug everything except what is needed for phono operation (turntable, prepro, amp, main speakers but not the sub). Remove the two Monster Cable surge protectors (some surge protectors can do funny things with grounds) and plug the prepro, turntable, and amp directly into a wall outlet. See if the problems are still present. If not, reconnect the remaining items one by one.
Also, just a thought but cable boxes have been known to cause hum in audio systems, especially if something is amiss with the ground connection of the cable outside the house.
Good luck. Regards,
-- Al