Hum is a very specific sound. You can't hear it in the midrange or tweeter. I am assuming this is what you are hearing with the following comments.
Don't worry about hum if you can only hear it 6" from the speaker. That is pretty common. But the fact that the addition of the preamp causes the hum to be so much more audible is an important clue. Another important clue is the fact that the hum is affected by the volume control. If this were a ground loop problem or anything caused by the AC line you would be hearing a buzz, which is audible in the tweeter.
You did not mention if the CDP was connected or not when the hum was noticed with the preamp. If it was connected, disconnect it and see if the hum is still there. If yes, return the preamp for service- the hum should not increase with the volume; this suggests a filter capacitor problem in the power supply of the preamp.
If the hum goes away when the CDP is disconnected, have the CDP serviced out. Digital devices can make all sorts of sounds including hum when things go awry.