From the EP-500 owner's manual ...

The subwoofer has an instantaneous standby mode that enables itself whenever there is no signal input from your A/V receiver or preamp. The Green LED POWER indicator does NOT change color when the subwoofer enters its idle mode, however power consumption is reduced to 10 watts or less. Leave the switch at the ON setting unless you go away for an extended period. Moving it to OFF will shut down all power to the subwoofer.