Brian is on spot, over the years I have done many sound systems even spending 6 months through a seminar at Electro-Voice at the Buchanan, Michigan plant to get my contractor certification to do installs for them, Most 60hz hums in a sound system can be traced back to grounded neutral or bonded neutral bus's in sub panels which is a common code violation, many times it's not even the circuits that are being used as a grounded neutral from even a different circuit will cause this, there are a couple methods for eliminating this hum and tracing down a bonded neutral can be the more expensive choice but the best for safety and for a code compliant install.
But like with some Church's and small establishments this can be over bearing for a customer to bare this much cost as it is very time involved and the cost meter can run up quit a bit.
I would say the first thing is to check if the subpanel has the neutral bar bonded to the ground, this is a code violation and should be addressed, always make sure you have a good neutral to this subpanel and a separate EGC (which can be metallic conduit) not service entrance cable as I have seen many times, if this checks out ok then you will have to turn off breakers one by one until you remove the hum leaving the last two circuits that feed the sound equipment on, if this doesn't remove the hum then I ask if this sub panel is fed from a different service as you might have a situation that the only way is to install a 1 to 1 isolation transformer in your audio cable from the sound board to the amps, at least one or two per Amp is needed, these can be purchased at most pro sound shops and many guitar shops, you can get them in 1/4" 600 ohm, or 1k ohm XLR in many configurations, I used to have a collection of them when I was doing sound, as they were a show saver many times over, the worst method is using a gate or a notch filter to take the 60hz out, as this will color the sound and make it sound like something is missing, a gate can sound awful cutting in and out when you have a soft voiced person at the mic.
the other things that can cause problems is what is DI'ed into the board or sound snake, DI'ing some guitar amps and effect racks can cause problem if one has a mis wired cord, older tube amps used one side of the power cord as a reference and usually had a switch to change which wire was referenced, this can cause problems, with DI'ing these into a PA system, so if you have this type of system then have them remove all the DI'ed equipment and hook in one at a time till you find the one that is causing the problem.
(DI-ing or DI-ed is slang for "Direct Input" or directly connecting the low level output of a guitar amp or other sound device to the main sound board)
These kinds of problems are not for the average electrician as it can require skills of knowing how the sound system all ties in, but parallel neutral current on the grounding is, and should be eliminated if possible, and is a very common source for this hum.
The reason parallel neutral current on grounding is a problem, is when we have amps on one circuit or receptacle and the sound board, or guitar amps, or other sound equipment at another location plugged into another circuit or receptacle any current on the grounding will also share on the shield of the connecting audio cables or sound snake, this is because current takes all paths, and even though the current on the snake will be small, 60hz is right in the audio spectrum that the amplifiers will amplify, and it does not take that much to produce a big hum in a system.
From the way you asked this question and the fact you tried using ground rods to eliminate it, kind of says you might need to pull in someone who knows how to deal with a problem like this, and watch out for sound guys as all they know is cut off the ground pin or use a adapter that removes the equipment ground connection as this is a very dangerous thing to do, isolating with a 1 to 1 transformer in the audio cables is a much better choice if you run into a brick wall with being allowed to track the problem down because of the cost.
I shouldn't really say that about sound guys as they are some sharp guys out there who did get some instruction on how to run sound systems, and I have trained some in this area also to stop them from cutting off ground pin on cords:rant:
