Detecting “Dirty” Power

I bought a used BHK 250 still under warranty. It is plugged into a Denali S power conditioner.  I heard a hum coming from the area around the tubes.  I cannot hear it from my sitting position and it doesn’t effect SQ, but I want it gone.  

I sent it to PS Audio; they tell me there is no hum and sent it back. They implied I mostly likely have a power problem and recommended some troubleshooting steps.  I unplugged everything from the 250 and plugged it straight into the PS Audio receptacle.  The hum is still there, but it is not as pronounced.  A P15 power regenerator might fix the problem, but I want to know if I have a power problem before I do anything.

How can I determine the quality of the power coming into my house and out of the receptacle? FYI, the Denali is plugged into a PS Audio receptacle/dedicated 20 amp line.  I have a whole house generator w/ transfer switch and whole house surge protector.