I suspect the problem is (as noted by Doc B) one of ground currents. A device that is not safety-grounded (3rd wire of the power cord) will have a ground voltage that is not the same as that of the Stereomour, whose signal ground is tied to safety ground. This will cause a pop when switched in or out.
You can test this by measuring the voltage difference between the RCA jack grounds on the two devices when they are NOT connected. It's almost certainly AC voltage, not DC - but always interesting to check. Usually the impedance behind that voltage difference is high, so the resulting current when the grounds are connected will be small - you can try to measure that as well, but it may be too small to measure easily. fortunately the current is so small that there is no damage to the switch or other components - unless the speakers are making excessive movements :^)
If you can, try reversing the 2-wire power plugs to see whether one orientation makes less voltage or current.
"Ground loop" hum is a result of that current, which is why we switch the grounds. Otherwise any source with a ground current will create hum even when that source is not selected. The alternative is to leave all grounds connected (unswitched), which should eliminate the pops but risks creating a new source of constant low-level hum. If the available ground current is small enough to not make a problem hum, then the alternative might actually work better, at least until a source with a noisier ground is added to the mix. You can get some clip-leads and use them to temporarily connect the grounds, checking to see if that does indeed stop the pops.
