My friend, thanks for your effort to put forth the best service you can for your customers and guests.

I think you may have shot yourself in the foot when you did away with the ground feature. Back in the day ... I grounded all devices (amp, pre amp, turn table, reel to reel tape deck, tuner, etc.) to each other. I ran a wire from the ground on my amp to the screw holding the outlet face plate in place (creating a electrical path to ground). I also plugged the turntable, etc. into the outlet on the amp. This reduced the possibility of reverse polarity conditions. Obviously this was in a home, not me running a business AND a different generation of equipment. Also at that time, I discovered the 60 cycle hummmmm and found the only thing I could do was reverse the ungrounded plug of the amp into the outlet.

There was a thing called a Hi-Pass filter that I could have added to get rid of any additional noise ... but it was pretty expensive at the time and I had already eliminated most of the noise.

You might want to invest a couple bucks in a polarity checker. Such device will tell you immediately if the power being supplied to you is proper and correct.

Here is a link to such a device: http://www.amazon.com/Instant-Outlet-Circuit-Tester-Indicators/dp/B0012DHVQ0/ref=sr_1_5?ie=UTF8&s=hi&qid=1262371973&sr=8-5.

Here is a link for a audio specific checker: http://www.vandenhul.com/p_IG01.aspx

These are of course not the only places to find these devices, and don't neglect to check the advertisers here at FixYa.com.

What has this to do with you ... maybe a few ideas you coud try to eliminate the irritating noise your guests are occasionally subject to. Noise filters are much more compact these days and way cheaper. You might want to explore a "isolation transformer" a device you plug in to the host which is suppose to feed you a clean source of power.

There is the possibility that some piece of gear you are using has a transient electrical problem. Since you are using mp3 music, (all on your computer, right?) I'm guessing you should look at your mixer.

There is also that possibility that the noise has nothing to do with your equipment, being generated at or near your gig site. How does that work? Every time there is a spark, there is a signal - it sounds like what you describe. It could be that some length of cable you are using is sympathetic to the particular frequency of the noise being generated locally. Move your cables - change the length. Filters available for your audio? You probably know more about that than I.

On a final note, surge protectors come in all flavors. There are surge protectors that are really only power strips, there are surge protectors that are pretty good at supressing surges and some noise, there are those that are great and there are those that at EXCELLENT ... the better ones are pretty pricy, but they work. You may want to inquire about such a device at a professional Electrical Supply place in your area.

(I have presumed you are a US resident and using 120 V AC while responding to your question)