Krell fpb 600 problem

I have owned krell fpb 600 for few years. Recently it ve got a problem when I turn on power, one side of speaker is mute except I turn of and keep standby mode at least 15 mins or if I keep standby mode all the time it will be ok. So in case I want to play, I need to wait at least 15 mins to warm up. Anybody can tell me the problem and how to fix it because no krell dealer around me

Thanks