I'm a little unclear about which plugs (left/right top/bottom) you're talking about. Referencing the image below, I take it you mean that the sub was originally plugged into the rightmost, orange outlet either top or bottom. If you can clarify what you mean by reference to the image, it'd help.
Further, you don't state whether the lights are on the same circuit or what kind of lights they are. I'm going to assume for the moment that they're incadescents.
My understanding on the way the outlets on the 5000 MKII work is that each vertical pair is 'isolated' from the adjacent pair by means of RFI/EMI circuitry. Therefore, looking at the back of the unit, I'd take it that as you move from right to left you get increasing amounts of noise attenuation (filtration). The fact that you can plug it into one set of the Monster outlets and get a pop while moving to the left renders it a non-issue, strongly suggests that the source of AC contamination comes via the AC line itself and isn't air born.
If for some reason you can't either relocate the rack your system is on or reposition it, then there's a couple of approaches you can take.
1) Replace the switches with newer ones that don't bounce. It's possible that the switches are bouncing and that's causing the problem. OTOH, there might be a wiring issue but it's hard to say.
2) Get an extension cord for the sub. That's kind of ugly though and I'd only use it as a temporary measure.
3) Replace the cord with a longer one, say 8 or 10 feet. Now if your sub takes a replaceable cord, this is a pretty easy approach and you'll find suitable replacements at MCM Electronics that are inexpensive.
4) If the cord is hardwired, one thing you could do is simply buy a longer cord of the right gauge and just switch cords. Just pay attention to keeping the color coding the same. Run the idea past the vendor of your sub.
Some tech support is just outright piss poor and amounts to nothing other than sending you to your dealer, sending you to a website, or sending you literature. That's what happens when they hire borgs.
