The first thing I would do is make sure the cables are routed correctly. It sounds like cables are plugged, dropped and forgot. 1/2 the time I can fix noise in a system with routing cables at 90 degree crossing and nothing closer than 1/2". Over, under or along side. I’ve done it that way for 45+ years. My systems are OLD and new all have no noise (not low) NO NOISE. You pick the source and turn it all the way up. If you stay still and don’t jump up and down in the room with the TT selected even that is quiet. ALL valves too. C20, ZP3, V12R. Dark background..
I’d look there first.. Tubes be careful what you have close to what.. Power supplies from Laptops and Cable boxes can be noisy too.
220/40 is an option too for the 901s and use the balanced side. What are you using for a Pre and Source?
Regards