Have you checked the speaker wire connections at the speakers and the amp? Shorted wires could be doing that.
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Seems like you need service. Also, see if you can get the wall outlets replaced. How many circuits do you have in the apartment ? Apartments in older buildings are notorious for having only one or two branch circuits for the whole place. If you system is on the same circuit with lights and appliances. that would explain why you are getting pops each time a switch is thrown. A line conditioner may help.