When an isolation transformer is not enough?
Finding the source of the noise is what is going to direct you to the "fix".  If the noise is from filter caps going bad in the amplifiers, time for new caps.  If the cabling doesn't have shielding to keep them from becoming antennas and "injecting" signals into your audio inputs, its time for proper cables. If this building is quite large, you are likely using main (not stage) lighting that can be quite noisy (Arc lamps) This noise can radiate into your inputs.  If you shut those lights off, does the buzz go away?  Are there any cheap "phone charger" type plug-in power adapters near the mixer?  Poorly made switch-mode power supplies like these can be a source of noise. If there is a laptop being used for any audio handling, unplug it power supply and see if you get a change while on battery power.

Because this started happening "over time" it can be due to caps going bad, but I am just speculating.