What does this tell me?

I've got a buzzing issue that I'm trying to track down, here's what I know.
(FYI, my amp is an integrated)

~It's only coming out of the tweeters of my Vandersteen 2ce's.
~It happens regardless of the source, DVD player, Squeezebox or DAC.
~It only happens if I select an input that has a source connected to it. ie, if I switch to an input that has nothing connected to it, there's no buzzing.
~I've tried every input and they all buzz when something is connected to that input and they all don't buzz if there's nothing connected to that input.
~I've tried different power cables and IC's, they don't change anything.

I don't know where to go from here. Is it the amp? Is this a ground issue? What do I try next?

Sounds like it could be a grounding issue. Please give details on the circuit you have this connected to, if there is anything else connected on the same circuit in the room (lights, dimmer switches, power-blocks for computers, cellphone charging, etc...). what are you using for a pre-amp or processor, i.e., what's everything connected to? Have you had an electrician check that the outlets on the circuit are wired and grounded correctly?
Try process of elimination. Unhook all sources that you mention and install them one at a time until you find what is causing it.
It's a dedicated 20 amp circuit, with two outlets about 8 feet apart. It was
installed by my dad, who is an electrician.

One outlet has a TRL power strip plugged into it. Into the Power strip is the
Squeezebox, Oppo DVD player and Behringer DEQ. I have a Krell integrated
plugged straight into the wall.

The other outlet has Power Wedge power conditioner plugged into it. Into
that conditioner is a plasma tv, satellite receiver and sometimes a playstation.

I also have a Vandersteen sub plugged straight into the second outlet.
Besides the other good suggestions which have been offered, I'd suggest that you try unplugging the Power Wedge (and therefore everything that is plugged into it), and also try plugging both the Krell integrated and the Vandersteen sub into the TRL power strip (assuming it has sufficient current-handling capability).

Both the plasma tv and the satellite receiver could be generating rf interference that may be coupling into the low level components. The problem could also be worsened or caused by ground offsets (differences in potential) between the different outlets, or resulting from having some things on the conditioner and some things not on the conditioner.

Also, I'd suggest taking a portable am radio, tuning it to the bottom of the band (around 550kHz), and walking around with it in the area of the system to see if it picks up significant rf interference. As was mentioned, both dimmer switches and compact fluorescent lighting (or full-size fluorescents, for that matter) can be potent rfi generators, especially if the fluorescents are nearing the point of failure.

Hope that helps,
-- Al
Does your interconnects or speaker wire run across any power cords? This happened to me when I had the Odyssey Stratos amps in my system.
Get some two prong adapters to see if it's a ground loop, try amp and/or sub first.
I unplugged the sub, still noise. I unplugged the Power Wedge with the tv, &
satellite receiver, still noise. I took a radio tuned to about 550kHz and walk
around with it, whenever I got near the side of the rack with the amp, Oppo and
Squeezebox, it went nuts. I then unplugged the Squeezebox, still noise. I then
unplugged the Oppo, still noise. All that leaves is the power cable going into the
amp, the interconnects going into the amp, the speaker cables coming out of it
and the amp itself.

I guess I'll try re-routing cables and see if that makes a difference. And I'll try
Samhar's idea of the two prong adapter.

Thanks for all the input, keep it coming.
With all of those things disconnected, does the noise still happen only when inputs are selected which have interconnects plugged into them (the interconnects being unconnected at the other end, if I understood your post correctly)?

-- Al
The interconnects are not disconnected. The source that the interconnect is
connected to, is unplugged from power.

The only time there is noise, is when an input is selected that has something
connected to it, whether there's power to whatever it's connected to or not.
In my system, I have a preamp with WBT Nextgen input jacks, and I use some interconnects with Eichmann bullet plugs. Since the jacks and bullet plugs do not have metal completely around their barrels, the connectors have to be twisted until the metal pin on the Eichmann barrel makes contact with the metal section on the Nextgen jack. If the metal isn't making contact, a buzz results.

Until I discovered this, I thought the interconnects were faulty, or that I had a system grounding issue.
I'm suspicious of the Squeezebox. Is it connected to a computer via ethernet or wirelessly? If via ethernet, try disconnecting the cable. If wirelessly, try removing power from whatever is wirelessly transmitting to it. And of course keep in mind that any of these devices can probably continue operating for some seconds after the power plug is pulled, running on capacitively stored energy.

-- Al
And unplug the Squeezebox's wallwart power supply from the ac outlet, if what you did was to disconnect the Squeezebox's power by unplugging the wallwart from it instead.
I think I've got it cured, although I'm still not exactly sure what was causing it.

I unplugged the Squeezebox from the wall and disconnected the ethernet, still buzzing. So I decided to disconnect, reroute and reconnect everything. I tried to cross lines as little as possible. That helped, but there was still a little noise, not as bad, but it was still there. I then remembered, I have a power cable with a choke. I installed it on the amp. Viola! No noise at all, gone! Finally.

Not sure if the amp was generating the noise or if it was just picking it up. I'll live with this for a few days and see how it goes.

Thanks everyone for your help.