You describe the problem as "hum", which is characteristically 60 cycle hum, but I am wondering if you mean "buzz", "hash", or "sizzle", which is rich in harmonic content compared to standard 60-cycle hum. Lighting systems for years have been notorious for "SCR sizzle" from the silicon-controlled rectifier (SCR) in the dimmer circuit (not the par cans).
If you have exhausted all other possibilities described above, you most likely will need to install a 1:1 AC power isolation transformer and power all sound system audio components through it. Power isolation transformers can be installed by an electrician, or you could buy one off the shelf, such as the
Furman IT-20, install it yourself and see if the situation improves.
The advantage of the Furman solution is that you could purchase it through a box house like Musicians Friend or Full Compass, and have 30 days to see if the issue is resolved (and potentially return it), whereas working with an electrical contractor forces the expenditure. There is also a possibility that you may need to upgrade the lighting dimmer system.
Prior to purchasing any solution you will need to calculate current (amps or amperage) requirements of the sound system by adding up all the amperage or wattage consumption of various devices in the sound system. Wattage (W) is voltage (V) multiplied by current (A). The Furman IT-20 is good for 20 amps. If your sound system has a potential of 30 amps usage, then you will need two units, and divide the load between the units.
