Whoa this thread has blown up a lot. Your system seems good enough to have low end filtering so unless if its a design flaw (highly unlikely) or a damaged piece (somewhat likely), the problem has to be at your end. Lets try and isolate the cause first:
1.) You said you were going to go and check on the display pieces at the store? How did that work out?
2.) As the situation has evolved, I am becoming more and more convinced that it is a ground issue. Is the problem in one speaker or all of them? If the static is occuring on all of them, then it is an issue with either your reciever or gnd. I'm not sure how the wiring is configured, but then test the setup on an individual speaker level and plug it into your computer (if the wire allows for it) and see if you hear the static.
I suspect you won't, but anyway this will at least tell you that the physical speakers and their wires are fine.
3.) Solutions: one of your phases giving a slightly low voltage (how low btw?) really won't affect you. As stated earlier, the built-in power supplies of these things are rather robust. It is POSSIBLE that if your voltage is too low, it might offset a filter's frequency range, but I find that to be highly improbable. To discount this as a possibility, just switch the phase to another one and see if the hissing persists.
As I said earlier, I am fairly confident that the issue is in your ground/neutral. The only solution is to get a high-end stabilizer or a line conditioner, pure sine wave UPS for it. You can complain to KESC, but I doubt that they'll do anything about it.
Before putting money into that though, first exclude all other possibilities. See that there isn't a problem with your piece (check store display). See that it isn't an issue with just one speaker. Connect it to your computer and confirm that the issue isn't with the speakers or their wiring. I'm not really sure how to check your receiver (take it to the store when you go to check the display piece, and also have them test it out with your unit). Flip your phases for your satisfaction.
I'm sorry I don't have a better answer. I'm not an expert in these matters, but I am fairly confident of the problem and (unfortunately expensive) solution. Go through all other possibilities and solutions that people have offered before making a financial commitment.