I brought out my SM500S last night after issues with another rig. It sounded great but ran really, really hot and during the second set, shut off. I moved to a different preamp/power amp, and later it turned back on, presumably after sufficient cooling. I could not feel any airflow from the side vent, which is where the hot air is supposed to go. Does anyone know if the fan is supposed to run all the time or if not, how it is switched on? The amp was made in 2000. Thanks.