Along the lines of what Jea48 said in an earlier post, disconnecting the interconnects from the inputs is not conclusive unless you put shorting plugs on them. Especially considering the M60's high input impedance (250 Kohms), it seems conceivable that the inputs are picking up airborne rfi and then amplifying it.
If you don't have shorting plugs handy, you could take rca cables (the shorter the better), plug them into the inputs, and at their other end stuff in some aluminum foil so that it shorts the center pin and the outer (ground) part of the connector together. Obviously, have the power off while you are doing that.
Regards,
-- Al
If you don't have shorting plugs handy, you could take rca cables (the shorter the better), plug them into the inputs, and at their other end stuff in some aluminum foil so that it shorts the center pin and the outer (ground) part of the connector together. Obviously, have the power off while you are doing that.
Regards,
-- Al