Good small/single outlet power conditioner for preamp.

@jriggy  I had the same issue when introducing one of my valve FM tuners to my system. A different tuner works perfectly.

I trialled the 156 and it didn't work in my system, altering piano sound too greatly. I have since enjoyed a UK built "AC-2K Balanced Mains Supply". This reduces noice floor and enhances clarity and impact in my system while not emasculating natural sound of piano or other instruments. Good luck.

Grounding issue? Maybe you need to use a cheat plug on the preamp power cord? 

Use the cheater plug to see if that is the problem. If that cures the problem find the reason for the ground loop and fix it. By using a cheater plug you are lifting the safety ground.

@jriggy why do you think you need a power conditioner at all? I had 156 and now running everything direct on 2 dedicated circuits.
Doesn’t answer the question why you get hum but eliminates the issue. So why not just run direct?

Try the Furman AC215a, it's small (2 outlets) and has the features erik squires recommends.  It's about $100.