Identify this hum, please

Added a new (used) mono pair of amps which didn't hum in the system I heard it in. Previous amps haven't hummed in my system at all. It hums equally from both channels, gets louder as you turn up the volume. Tried lifting grounds with cheater plug, no change. Tried moving it to a different circuit using an extension cord and it hums and buzzes there. What could it be?
Are your preamp and cd on the same electrical circuits?
Is there a cable connection (from tv) that is routed through your system?
Tried different interconnects?
Its not the amps if the volume control makes a difference. Does it hum if the volume is all the way down?
Try the cheater plug on the preamp and also try a different set of interconnects from the preamp to the amps.
Connect your source (CD or TT) to pre-amp, connect your pre-amp to the amp and connect the amp to the speakers. Hum? yes? then turn off components and disconnect your source (CD or TT) from the pre-amp. Turn on components. hum? no? then it is the source equipment or the interconnect cables from the source to the pre-amp or the power connection of the source. Hum? yes, then turn off components and disconnect the pre-amp from the amp. turn on amp. Hum? no? then it is the pre-amp or cables from pre-amp to amp or the power cable connection of the pre-amp.

Always start with everything connected and hum present, then work from the source-pre-amp-amp in that direction to eliminate the problem. Remember, it might well be the interconnect cables. Some connect signal ground to the shield and may cause ground loops also.

Under no circumstance do you ever use a cheater plug. I can't tell you how many times I see people suggesting the use of cheater plugs that have no idea that they are actually suggesting that you violate a basic safety issue and may cause serious injury to you, your family, pets, etc. Do not ever use cheater plug. Find the problem and fix or remove it instead. After you have taken the steps listed previously to find the faulty piece of equipment or connection, connect all of your low level signal sources (CD, TT, Pre-amp, Tuner) into the same power conditioner or outlet block which then plugs into its own wall outlet directly to the panel. Then plug your amps into their own separate outlets that go directly to the panel. So, at minimum, two totally separate outlets wired straight to the electric panel. If you have more than one amp (bi-amp) then separate outlets for each amp wired to the electric panels. In this case, three dedicated outlets wired individually to the electric panel. Noise floor significantly drops and no ground loops.

No cheater plugs. This is like someone telling you to drive with no hands. Sooner or later you will run into something.

It's not the amps. I hooked my iPod up to the amp and listened and there's no problem. It's both the preamp and the phono stage since when I unplug the phono stage/turntable (which is powered by the same DC power supply -- WT Simplex and a Dynavector P75) much of the hum goes away but there's still a buzz so that's the Lighter Note LDR preamp that I'm using. So the question is: Is there anyway to isolate the preamp side from the amp side? Putting the mono amps on a different circuit doesn't help at all. Same exact issues. It's a 60 year old house with some interesting wiring issues. The thing is, every other amp I've ever had was not a problem. Thanks everyone for the help.
It sounds to me as if you have a ground loop. This is likely caused by poor wiring practice in both the amps and the preamp working in tandem.

The wiring problem has to do with how the units are grounded through the power cable. Although Minorl is correct in his admonishment to not lift the safety grounds, I do understand the temptation to do so.

Here is my recommendation:

Go to the hardware store and get some of those ground cheater plugs. You are going to do a test to find out where the problem is.

Start by using the cheater on the preamp.

If hum goes away you have two solutions:
1) send the preamp back for repair to the ground
2) get an isolation transformer for it

Second, plug the preamp into the wall normally and put the cheater on the amps.

if hum goes away you have the same two solutions as listed above.

DO NOT use the cheater otherwise! It represents a shock/fire hazard should anything go wrong.

Alternative approach using a Digial VoltMeter (DVM):

Set the meter to ohms scale. Measure from the ground pin of the power cord to the ground of the RCAs.

If you read near zero ohms, follow the solutions above. If you read significantly higher than zero ohms (25 ohms or more), the unit is off the hook.
I agree with Atmasphere's approach.
Many ground hums are caused by Cable TV. Do you have any type of cable TV connection into any of your equipment?
I did all the ground lifting and measuring and in the end it's not the amp. It's the preamp. I replaced the passive with an old Advent 300 receiver (using it's preamp out) and the hum is gone. That Advent actually sounds WAY better than it should. I've always been a passive guy but I've heard that my amps (Joule-Electra Stargate SET) rather like a regular preamp. After hearing even the Advent I can see why. These are great amps by the way. Any preamp suggestions :)

Thanks to everyone for their help!
Ah. The hum was caused by noise pickup in your cables, and the impedance of the passive setup was so high it could not control it.

A tube preamp would of course be my first suggestion.
That's where I'm headed. Lots of money though so I'll be saving up!