Hum hum hum

nimbus109,
As you can see, there are many way to interpret what you have said. Please be as detailed and descriptive as you can to insure you are understood, with not miscues. 

Based on the comments so far, the problem is in the preamp (which is to say the cables and amp ar off the hook), but its unclear whether its a problem with a functioning source connected, as Al is asking about.

Since its affected by volume, its ahead of the volume control. The thing is, without an input, this could be perfectly normal. So connect something to the input and see if it still does it.

If you have no inputs connected to the pre you will most lokely pickup noise or hum. If you use a shorting cap does the hum go away. If you connect a device to the input of the pre is there still hum? If not there is no problem
Alan

Sorry I have not been clear. With nothing connected to the preamp, I merely switched the xlr cables at the preamp end. Should I have switched them at the amp end as well? What are shorting caps? I was getting hum on one channel when I had cd and turntable connected and turning up volume with nothing playing the hum would gradually increase at higher settings

If you search at eBay for "RCA shorting plugs" you'll find many examples, although I'm not aware of any that are readily available for XLR inputs.  The point to assessing the hum with shorted inputs (i.e., with a connection between the center pin and the ground sleeve of an RCA input connector) is that the typically high impedance of unconnected inputs makes them susceptible to pickup of hum or noise even if everything is working properly. 

However, since you've found that the hum is present with two different source components connected there's probably no point to trying shorting plugs.  And since the problem is sensitive to the position of the volume control the XLR cables and the amp are ruled out as suspects, as Ralph (Atmasphere) indicated.  So everything seems to point to the problem being in the preamp.  Assuming, that is, that the symptoms when the cd player is selected are essentially identical to when the phono source is selected.

Regards,
-- Al