The presence of a 60Hz hum in the speaker only when a connection is made to another piece of equipment suggests the presence of a ground loop. Such a loop is caused by electrical interaction of the signal grounds on two pieces of equipment, and is not necessarily an indication of broken gear. If a balanced connection to the SRM450 is being used, the simplest way to correct this problem is to lift the signal ground on the end of the connecting cable closest to the signal source. NOTE: This is not to be confused with disconnecting the safety ground on the power plug - this should be avoided at all costs, as it can present a safety hazard. If an unbalanced connection is being made to the SRM450, a line coupler such as the Ebtech Hum Eliminator can be used to eliminate the ground loop.