Hello Dan,
I would recommend downloading a tone generator on your smart phone and/or computer.
With the tone generator, set it to generate a 60Hz signal, then feed that into the Quickie.
Set your meter to AC volts (if you don't have an autoranging meter, set to the 20V range or the closest similar setting).
With the 60Hz playing, you should be able to measure AC voltage at:
T1, T6, output jack left solder cup, output jack right center cup, A/B6, A/B4, A/B2 (greatest AC voltage)
B side potentiometer solder lugs (second to greatest AC voltage)
A3/B3 (no greater than the reading above, but equal to if the volume is all the way up)
I would use your meter to measure the voltage coming out of your phone or computer, do your best to get it up to 0.5V or more, that will make it easier to measure. Start at the RCA jack of the offending channel, verifying that the voltage is present, then measure at the output of the selector switch (the forward two lugs in the middle), then keep following the signal until you get a zero, then let us know where that is.
-PB