Hum from Subwoofers

I've noticed a hum when the volume of my system is turned up (with nothing playing).  The hum is silent when I unplug the subs (a pair of REL Storm III's).  I tried using cheater plugs on the subs, but it makes no difference.  Hum is present with the ground lifted (with cheater plugs), and present with the ground connected (without cheater plugs).  Again, when I unplug the subs, the hum goes away.
The subs are plugged directly into the wall outlets, and the rest of my system is plugged in to an Audio Magic Mini Ref conditioner (no unused outlets left on the condioner).
Any thoughts on how to correct this problem?  
In advance, thanks for your input!
The question is if the hum is proportional to your volume control. If it is, then the hum is at least from there. It may be that your speakers don't have enough output at 60 Hz without the subs. 

Turn everything off and remove from the power everything but pre-amp and sub. Still happen? 


One forgotten source of ground loops is external coaxial cables, like to your Cable TV or Satellite dish. Make sure you unplug those too. They make inexpensive GL isolators but they depend on your devices. 


Thanks for the responses Eric.  1.  The speakers are full range (Legacy Focus SE's), and yes the hum is proportional to the volume.  So are you saying the hum is coming from the sources?  I get hum from both my to TT & my CDP.
 2.  No TV in the room.
Sounds like your sources are picking up stray mains hum, possibly from transformers or motors that are being amplified by the subs. I'd try running everything without the conditioner. Plug it out and move it away.

Forget the cheater plug !!!! You have a ground loop hum.. I do not know if you are using the Hi or low level inputs nor do I know what amp you are using (class D or A/AB)... JUST REMEMBER !! a power company  distribution is X voltage @ 60Hz. Let me know more facts and I might be able to solve your problem.
Rkilowatt- on both the subs I am using the Hi level speakon inputs.  The amps I am using are Clayton M-100's (Class A, 100 watt mono blocks).  The pre-amp is an Audible Illusions L3.  The speakers are Legacy Audio Focus SE's.
The hum is proportional to the volume (i.e., more volume-more hum; less volume-less hum).  The hum is not noticeable until the volume reaches about 12:00 (which is loud).
All suggestions are greatly appreciated!
Float the black wire on the REL speakon connection at the amps(Both amps)Please read REL/Support> Monoblocks (2rel) connection method.Read first and then experiment.......Good luck!!!!
Rkilowatt,  I am using a REL Speakon cable from 'Signal Cable'.  I don't understand what you mean by "float the black wire on the REL Speakon connection at the amps".  My cable only has two cables at the amp end.  A Red and a Black, terminated via banana connectors.  I have the Red cable inserted in to the positive speaker output, and the black wire inserted in to the negative speaker output.  The Speakon connector is connected to the unbalanced hi-input at the sub.
 Let me go back to the REL Manual, and review their instructions regarding connection from a mono block amp to the sub.  Again, I am using two subs.  This hum is maddening!😲
Do you have a phone number ????