A Q about isolated ground outlet & humming amp

Hi
Every day around 7 or 8pm my power amp (Pass X250)starts humming.I suspect that somebody gets home from work in my building and turns some kind of appliance on that leaks DC in AC line.The hum stops somewhere very late at night,2 or 3 am (i never had a chance to stay that late to find out)In the morning at 8am and till 7 pm it is dead quiet then it starts all over again.I had a heavy duty hospital grade outlet.Yesterday i installed a heavy duty Isolated Ground outlet in hope to defeat the hum.The sound really improved due to a 20 amp outlet contact tightness with the power cord but the hum appeared in the evening as usual.
My question is:
What is the purpose of isolated ground outlet ?Nelson Pass warns in the manual never to float the ground.Am i floating the ground by having IG outlet?
I connected the outlet the usual way like all regular outlets are connected.There is a ground screw on the side but i have nothing to attach to it.Please,help me in understanding the whole thing.Thank You.