Left channel hum in Extended FPIII

[Comfortably Dumb]

I have Goldpoint selector switch and mono attenuators (wired in shunt mode) in my Extended FPIII and am experiencing a loud hum in only the left channel under certain scenarios.  It seems related to having more than one source plugged into the FPIII at one time.  I am experiencing the following:

1.) Having only one source plugged into input "1" - No hum in either channel

2.) If I have two sources with one in input "1" and the other in either "2" or "3"- No hum if selector is at input "1", but I get loud hum in only the left channel if I switch to input "2" or "3".

Also, with a source playing in input "1", I notice that I can hear the input "1" in the left channel even when the selector is at input "2" or "3".

FWIW, I did not have this problem before I did the Extended upgrade? 

Any ideas what is going on here?

Thanks and Happy New Year!

[Grainger49]

One of the many enhancements on the FP III is that it switches both hot and ground of the inputs.  When not selected the ground of an input, well all not selected, are not connected to the FP III at all.  It sounds like a wiring problem with your input 2 and/or 3.  Those should be traced from the input jack through the selector switch. 

The extended upgrade gives a little more gain but doesn't physically do anything to the inputs. 

This one is a head scratcher.