Ground Loop Problem - Stumped

I've got a nasty ground loop problem that I can't seem to figure out. I've done extensive research and tried just about everything but to no avail. Here is how I have everything connected. I am using the ExactPower EP15A and SP15A. All of my sources both digital and analog are plugged into the SP15A which in turn is plugged into the EP15A. My amplifier is plugged into the EP15A. The EP15A is plugged into my wall outlet.

1. I removed all the interconnects between all the components but kept the speakers connected - NO HUM.

2. Connected my preamp (Meridian G68) to my amp (EAD PowerMaster 8300) via RCA interconnects - HUM.

3. The hum is not affected by volume level.

Any ideas?

I'd try using a cheater plug with my amp. That should do the trick.
If you plug the amp and preamp directly into the same wall outlet do you have the hum?
Or what happens if you plug both of the Exact Power units into the wall, rather than one into the other?
Have you tried an isolation transformer between your preamp and amp?
Is any part of the system connected to TV cable?
Is it a ground loop or is it 60Hz pickup on an input?
">>All of my sources both digital and analog are plugged into the SP15A which in turn is plugged into the EP15A. My amplifier is plugged into the EP15A. The EP15A is plugged into my wall outlet."<<
The SP15 is a balanced power output isol xfmr unit, 60/120V.
The EP15 output, I believe, is 0/120V output unit. Is that correct? Imo there is your ground loop problem. Preamp fed from balanced power, 60/120V, and power Amp 0/120V.
Just a thought.....
Is this a recent occurance? Did you change something or add acomponent or has it been there for awhile? I would start by lifting the grounds on the components to determine which component or components is causing the ground loop.

The EP15 is floating the ground from your electrical outlet. This is causing the hum. Call exact power and see if they have a mod for this problem or if they give you some procedure for grounding the unit.
I Haven't the time right now to read all the responses, so forgive me if this has already been addressed. The hum you describe is likely 120hz(full wave rectified 60hz line freq) picked up from the power supply. The difference in voltage drops between your two grounds,the innterconnects, and the power cords,cause this noise. If only one power ground,generally at the preamp, is used ground loops can be avoided. HOWEVER, if the interconnects come off you gots an ungrounded piece of equipment. Not good.
Are you sure your interconnects are good? You may want to get a meter and check continuity.
Try grounding your amplifier to your preamp. You would need to losen a screw that attaches to the frame of your amp and preamp and attach a ground wire. I have the same problem and talked with John Bayloff @ Theta the other day and that is what he recommends to do. He suggest maybe doing it at the feet of the unit. I have a Meridian 568.2 and Theta Dreadnaught. It only occurs on the rca interconnects for the surrounds as I use balance cables for the L/C/R channels. I imagine that your EAD amp is balanced and the RCA cable is unbalanced only which creates the 60 hz ground loop. Good Luck
Thanks for all the responses. Sorry for not following up, I've been swamped at work and haven't had a chance to enjoy my audio system. Okay, I will answer each of the questions:

1. Plugging the map directly into the wall does not alleviate the problem. I still get the hum.

2. Plugging each of the ExactPower units into the wall rather than into one another does not fix the problem.

3. I have not tried an isolation transformer between my amp and preamp.

4. My system is connect to cable TV. I have disconnected all the cable TV connections and the problem still persists.
It is important to note that this occurs with everything else disconnected and unplugged from the outlet. All I need is the amp and pramp connected and powered on and connected to the speakers.

5. I don't know if it's a ground loop or a 60Hz pickup on an input. One interesting thing is that I just got some Odyssey Mono Amps and when using those to drive my main speakers I don't get a hum at all from the main speakers. I still get the hum from the center and surrounds though that are connected to the EAD amp.

6. I've tried plugging everything into just one of the ExactPower units and the problem is still there.

7. I have tried 3 different pairs/brands of interconnects and the problem persists.

So it seems odd to me that with another map I don't see this problem. Could it be a problem with the amp? If so, how do I go about diagnosing what the problem is if any before I ship this beast to be serviced/checked out.

Thanks again to all who replied.
What G68 are you trying to get to work? If it is the XLR 'balanced' output version ("G68XXV")and you are using truly balanced interconnects into a truly balanced amp input, then this may be the problem. I have heard the rumor that the XLR connectors are NOT configured by Meridian in a balanced manner and so a balanced interconnect does not work properly. I am currently investigating with Meridian and will keep you posted.