Good small/single outlet power conditioner for preamp.

If you plug the new pre-amp into your power amp with no source devices attached (while connected to the PSM156) - any noise from just the pre-amp and power amp?

If so, try a different set of interconnects? Are you using RCA or XLR cables to connect the Pre to the Power? Does the Pre-amp have a three prong power cord or two prong? If all is silent, then add one device at a time and see if one device triggers the hum. I have never seen a Puritan cause noise.

@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?