Tube PHONO preamp interference - RFI, EMI, bad grounding?

Although I don’t recommend tube rolling in order to improve sound quality, in this case it might be worthwhile to change all the tubes in the unit. It could be that one or more tubes is defective, thereby enhancing susceptibility to the noise. Even if that did not help, there is also the possibility that there is a cold solder joint somewhere in the chassis, possibly in a connection to a tube socket. So I would try the inexpensive and simple things first like shorting the unused inputs, and shielding the unit, and shielding the cables, but if that does not work you might want to take it to a knowledgeable technician who can measure the voltages. 

@theaudioamp thanks, this is nice. 50hz means a ground loop, right?

the system is overall quiet with the other inputs. there's only slight white noise on the DAC's input, audible only when I'm really close to a speaker and at high volume levels.

If it was in the building wiring the noise would be gone with that battery we have tried here. 

@lewm I'll have a different set of tubes next week. It's gonna take a while to get the shorting plugs.

I still think the EAR preamp is not defective simply because it works well anywhere else I have taken it. This must be an issue of the location. Soon I'll bring a simple integrated and plug it in in the hallway two floors lower. 

@filipm ,

I am suggesting that your phono stage is picking up RF that is being radiated through the building wiring. using the inverter would not change that. Though I assume that your turntable was unplugged from the AC and/or unplugged from your phono stage when you did that test.

@theaudioamp That's possible but the noise if being picked up even when the turntable was unplugged from everything. The signal was just weaker. I can plug any RCA cable to the input of the phono stage to get back the strength of the signal, doesn't matter if the turntable is plugged in on the other side. That's the most confusing. 

I'll know more next week though. Different tubes, I'll do tests around the building with more practical intergrated amp (I'm using separates)  and I'll have the two closest neighbors' wifi switched off.

@filipm , an underlying 50Hz, with a 2KHz packet rate is not WiFi. WiFi is rarely an issue with interference into low frequency analog electronics. The 2.4GHz signal is fast enough that most "things" cannot demodulate the frequency, and the modulation method also helps. I strongly suspect it is something else, though it could be 2 somethings, the 50Hz from building wiring, and the 2Khz from something else. Some home automation could be close enough to a 2KHz tone.

@filipm can you please do the following if you have a multimeter. Measure the resistance from the signal ground on the Phono input to the metal case of the box.

CABLES: I don’t know all the cables you tried, but you don’t need BS marketing crap like how the AQ Mackenzie is described. You need good old fashioned doubled shielded coax. Ignore all the marketing crap and use something that is designed using proper engineering principles like the Blue Jean LC-1. https://www.bluejeanscable.com/store/audio/LC1-design-notes.htm   A cheap $10.00 coaxial 75 ohm video cable is likely to work better than that AQ crap for rejecting RF noise.

If adding the RCA cable (without the turntable) brings back the noise completely that tells me that your probably not using coax and/or (could be both), there is no connection from the signal ground to the metal enclosure. There should be a connection, at least capacitive between the two, so that the "cage" is unbroken and so that the potential (voltage) between the two is the same at RF frequencies so that no noise can transfer between the two.