Hi Creamman,
first I have to say that I don't know this project from Drip or have built it myself before.
I have the following ideas and questions.
You write in your first post, that from your point of view, it is not a grounding problem. I think that is exactly what we have here.
The 90Hz you measured is most likely 100Hz since we both live in a 50Hz country. That and the fact that the hum is independent of the gain and other settings suggests that you have a groudloop somewhere. That happens very easily in such a complex project.
Therefore rule number 1: Each point of the circuit must have only one way to ground! Check this again. This happens often at the xlr jacks or the mounting of the PCB via the housing, provided that this PCB is error-free and has not already integrated this in the (signal) routing.
Since your hum is equally present on both channels, you should also take a closer look at the power supplies and the transformer.
When I compare your setup/the PCB with the schematics, it is noticeable that many things are not mentioned in the schematics. Are there more schematics of the Drip PSUs? How is the DC heating realized? Is the heater voltage referenced to B+? Elevated heater? Which transformer? 2 windings for the heater? How are the relays powered? Is there a central ground point? How is the protective earth connected?
Again, check for ground loops here, each part of the circuit may only have one path to ground. Are all voltages correct? Do you have a proper oscilloscope with which you can check the individual voltages?
Good luck!