HELP Kef PSW 2150 trouble shooting

I have a Kef psw2150 powered sub that will not work properly. When switched on there is a loud bass pop and then it will play for about 20-30 sec then no sound. Then when I switch it off it will start to play again till it fades out which Is about 15 seconds. I have another sub that works just fine so I know it's not the input. Need help trouble shooting problem. I have basic knowledge of electronics just don't know where to start or how to diagnose the problem. I did tap the relay and hold a magnet to it so far and no change. It's also hard to find a schematic for it but I will post pics if someone is willing to give advice. PLEASE HELP...