I'm reporting this because it might be useful to someone.
I'm powering an electric piano and an Amp4 from the same source, which produces an earth loop due to this common power source. The problem is that the signal coming from the piano is referenced to ground at the piano, and the amplifier input is referenced to ground at the amplifier, but there is a varying voltage difference between these grounds due to the varying supply current to the piano and the resistance of the cable in between. This is superimposed on the signal and results in nasty buzzes and clicks.
The proper way to overcome this is to have a balanced input to the amplifier, and to connect the "cold" side of the input to ground at the piano end. Both piano output and amplifier input are then referenced to the same voltage. But this requires extra circuitry (transformers or op-amps) because the Tripath chips (or at least the TC2000) don't have the right connections to their input op-amps available to construct balanced inputs.
However, what they do have is a decoupling capacitor for the bias generation circuitry, which on the TC2000 used in the Amp4 is pin 1, BIASCAP. (I presume at least some of the amps which use a different chipset will have the same thing.) This provides the reference for AC signals to the "virtual earth" which the input op-amps use, but doesn't carry any supply current to the chip. What I did was disconnect the bottom of the bias capacitor (C7 for Amp4) from ground, and connect it to the ground side of the input signal lead, which then has no connection to local ground, and hence no piano supply current flowing through it, and hence no voltage drop, and hence the interference is greatly reduced. (It doesn't disappear completely but there may be other factors coming into play). I also connected a 100 ohm resistor between the bottom of the capacitor and local ground to maintain the reference and so stop anything horrible happening when there is no input lead connected.