Good small/single outlet power conditioner for preamp.

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.

@gramophone_canada No, there is no noise when just pre and power are connected. Noise comes as soon as a DAC (two, tube or ss here to try) is connected.
I am using RCA from pre to power (and DAC to pre).
I did use a cheater plug on the DAC and it fixed the issue BUT a cheater plug is not ideal in this system. I could hear it’s affects (maybe I just need to let it settle), it added a harsh-brightness. And a cheater plug was not necessary on the DAC before this. I don’t know if it works the same with cheater plug on the pre, though.

Id actually like to not have to buy a second small filter and would rather have one that everything works with. I’ve never had hum in this system before with anything until now.

Today I am going to try the whole system on a Furutech tp609 passive power strip to see if it’s hum free. If so, I’ll dismantle the HT/TV power setup and try the Equi=Core 1800 balanced power conditioner with everything.

@psf4972 thanks for the suggestion. I looked at it but it appears to be UK based only…

wish I could edit thread title to reflect my change of heart on having two conditioners. One for all OR better yet, fixing this issue with no other purchase needed. 
 

Find a couple of PS Audio, Ultimate Outlets... make sure they are the high current model. One of the few power conditioner/tweaks I have a purchase that truly worked. Will never sell mine.