Andy said it was a ground loop problem and that we needed to lift the ground from one of the amps. I experimented with a ground-lift plug and found that I had to lift the ground from the originating amp. 10 I chose the THD Hot Plate because of the sound quality. It is very transparant and does not color or change the tone at all. I have tried the Marshall Power Brake and Scholtz Powersoak and both seem to color the tone.
All attenuators seem to sound best when still sending enough signal to push the speakers.
However the neighbors will appreciate the -16db setting as will your ears. It does have a separate low and high boost circuit that you can use at lower volumes to recover some of the lost tone. It also has a noise reduction circuit that you can use at all settings. The only thing that will need periodic maintenence is the zenon bulb that it uses for noise reduction circuit. If the bulb burns out you can still use the unit, but will not be able to use the built in noise reduction.