Buzz or hum

I have a hum in my system, Will Vincent amp, PS audio preamp with gain. and Coincident speakers.The buzz does not go up with the volume. When I unplug the RCAs from the preamp that go into the amp the buzz stops. I also plugged in my CD player directly in the amp and there was no buzz. So it is somewhat isolated to the preamp, I guess?
Someone said I might have a grounding issue but how would I fix that?

Thanks