First let us lay some groundwork: audio systems can exhibit "hum" and they can exhibit "buzz," which are two separate situations. To solve the problem, you need to determine whether your system is exhibiting hum or buzz.
Sixty(60)-hertz hum (fifty(50) hertz internationally) is a result of having a ground loop in the audio system. This is where there are two or more ground references in the system, and current is flowing from one ground point to another. Any piece audio equipment requires one ground reference. Ground loops can be formed in a number of ways. For example: An audio power amplifier obtains its ground from the AC power cord. The mixer, which drives the power amplifier, also receives its ground from the AC power cord. When the audio cable connects the mixer to the power amplifier the amplifier now sees a second ground from the mixer. If the mixer and power amplifier are both plugged into the same AC power strip then the mixer/amplifier interconnect cable shield can be cut to eliminate this problem. On most Crown amplifiers there is a "ground lift" switch on the back of the amplifier that performs this function and can be used to eliminate hum caused by ground loops. If you are using a cable service, such as cable TV, and you are routing the audio through your stereo system, you may experience a ground loop hum. This is a result of the cable company's ground reference setting different than your system ground reference. You can contact your cable company and get an isolation transformer that will take care of the problem.
Another cause of system "hum" is electrically induced, such as having a very sensitive component too close to a power transformer. Power amplifiers have large power transformers and can induce a magnetic field into other equipment. If you suspect this may be the cause of your problem then placing more distance between the two components is the only practical solution.
Excessive "noise" on the AC mains can cause "buzz" in certain components. Lighting dimmer packs are notorious for inducing noise onto the AC mains. If this is your problem try putting the lighting system on a different AC mains feed.
Ground loops, induced hum, and all kinds of nasty noises are sometimes hard to pin point. You may have to try several different approaches before arriving at solution. For more information call the Crown Technical Support Group and ask for the Sound & Communications reprint of "The AC Connection."