Check the area for any nearby fluorescent lights. All lights should be kept a minimum of 20 feet away from the amp as they can cause interference with the amplifier.
Look for any loose or broken wires connected to the amp. Loose wires can cause a poor connection, resulting in a hissing sound.
Ensure that the amplifier is on a flat surface. If the amplifier moves during a performance, the movement can loosen the wires or create interference by rattling on the floor.
Check to see if you have a Theta amp. These amps have a Decimator Noise Reduction System that will reduce the hissing heard from some amps.
Lower the gain by turning the knob counterclockwise to reduce the amount of gain introduced into the amplifier. You can also turn the volume knob slightly to reduce the amount of distortion that may occur.