PS Audio HCA-2: I sent a request on their tech support forum and was told by Paul McGowan who responded to the post to install a 1Kohm resistor in line with the black wire and the negative speaker terminal (I had tried the Ayre chassis screw trick and it did not work).Good info, and a clever solution if connecting black to chassis does not work (which I think would mean that on that amp chassis is not common with circuit ground).
If that proves to be necessary, with a 30W amp I'd suggest that the 1K resistor be rated for 1 watt (or more), to be sure that the resistor will not be overheated if the amp is putting out high power for significant time AND if a connection somehow exists (now or in the future) between sub ground and amp ground (which would result in significant current flowing through the resistor).
Regards,
-- Al