Krell KAV500 slight hum when connected to grounded plug

I have a Krell KAV500 with a slight hum when connected to grounded plug but totally silent when connected to a ungrounded one. I know a cheater plug could be used but I want to try and fix problem if possible since I plan on selling and dont want any problems. Anyone run into this issue with this amp? I have changed out the rca cables also and if you use a bad one you get a loud noticable hum in channel a bad one is used in.
 
I have a Krell KAV500 with a slight hum when connected to grounded plug but totally silent when connected to a ungrounded one. I know a cheater plug could be used but I want to try and fix problem if possible since I plan on selling and dont want any problems. Anyone run into this issue with this amp? I have changed out the rca cables also and if you use a bad one you get a loud noticable hum in channel a bad one is used in.
I have no experience of the amp mentioned, but have encountered this problem many times.
Are any of the auxilliary units (Tuner, Turntable, CD etc) grounded (we say "earthed" in UK) if so, a Ground loop will be causing this phenomenon.
I like to ground the Amp and ground everything else to the ground terminal at the back of it.