Hissing and Hum through speakers

You might try plugging the whole system into another circuit to see if that changes anything.

I don't think that your results with nothing connected to the amp are meaningful, unless you connect shorting plugs to the inputs (which will probably kill both the hiss and the hum, especially given that you initially reported that the noise went away when you had the preamp connected and muted).

As an experiment, I'd suggest connecting the preamp to the amp, a source component to the preamp, and the ac cords of ALL of these components either to the Panamax (if it can handle the current requirements), or else to the wall without the Panamax. That will eliminate the possibility that some or all of your problem is caused by voltage differentials between the grounds/chassis of the different components.

Not sure what you meant by "there possibly seems to be some level matching issues with the 561 (45 Ohms output impedance) and the amp (23.5K ohms input impedance on the rca inputs.)" Ideally preamp output impedance should be as low as possible, and power amp input impedance as high as possible. Those numbers sound fine, although the amp's input impedance is probably lower than typical, which might result in signal loss if the preamp output impedance were unusually high (which it is not).

Regards,
-- Al

Also, you mentioned that you moved the plasma to another circuit. Did you also try unplugging it? If it is physically close, it's possible that it is radiating high frequency interference into the audio system through the air, regardless of what circuit it is plugged into.

Regards,
-- Al

Thanks for your response...I did unplug the Plasma...it is also located in a different room than my equipment is (about 20' away from on the other side of the wall.)

I will try the experiment with 1 source, preamp and amp only connected.

Thanks again...