Having trouble with my PSW 300 sub. It stays on all the time! At first I thought it was getting some sort of signal from my receiver (sony strd-1000es) even if the receiver was off. But If I unplug it at the sub input it will shut down in about 5 min. If I leave it plugged in at the sub input and unplug it from the receiver it just stays on. How do I have this repaired. Is there a part I can replace myself.