I use this amp for my main home theater. On turn off via trigger circuitry from my Marantz AV7702, this amp causes the front left speaker (and only this speaker) to make a violent thump. The speaker cones move way outwards. I have turned the amp off via the amps own power p/b and the same thing happens. I think the power supply may be discharging through that one speaker. Is there a power down relay or some other component that has failed? Need some ideas.