Hum hum hum

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
 

Disconnected all inputs and I get a distinct hum at high levels on one speaker with just the amp/preamp/speaker connection....  I probably will never play at these levels

I think you are saying that the hum occurs at high settings of the volume control, and varies with the setting of the volume control.  Correct?

When I switch xlr cables, the hum goes to the other speaker.

When you say "switch xlr cables" do you mean that both ends of each cable are moved from one channel to the other?  Or that just one end of the two cables is switched between channels?

Disconnected all inputs and I get a distinct hum at high levels on one speaker with just the amp/preamp/speaker connection.

Does the hum still occur when a powered up source component is connected to the preamp, and that component is selected by the preamp's input select switch?

Also, assuming that the preamp and amp have 3-prong power plugs it may be informative to temporarily try a cheater plug (a 3-prong to 2-prong adapter, with the safety ground left unconnected) on the power plug of one of those components.

Regards,
-- Al