Good small/single outlet power conditioner for preamp.

It's the preamp. you changed 1 thing, problem started.

Preamp straight into the wall, no problem, thus the preamp is good, EXCEPT it is making .....?

I would get a length of wire for a new ground somewhere: start touching contacts, this one, that one: searching for some combo that ends the hum.

I did that on my new equalizer which hummed even in bypass mode, found connecting the left and right channel's grounds to each other solved the problem. Just hit or miss.

@elliottbnewcombjr Im not qualified/knowledgeable enough to do that. If I knew more to what you’re referring to, maybe so… but that sounds iffy for me. 
 

Maybe the new/different/unconventional design of the Puritan filtering system does not jive with the different/special design of the GreenForce power supply in this preamp. 

Take a look at the Audience AR2P conditioner - it has surge protection and does a nice job cleaning up the AC.  Should solve the problem with the preamp.

have you asked the equip makers/dealers for help/ideas?

@elliottbnewcombjr yes, the Preamp maker has been very helpful, guiding me through lots of these checks.