Humm from phono but not dc or aux ?

Hum or RF? Two very different things that generally also sound different.

Right. Next Q: how faint?

No, you need to check the ground on your turntable and tonearm. Some cartridges are prone to humming. Grado has a history of this. If you crank the volume all the way up there will always be at least a faint noise on the phone input because of the high gain. With less expensive equipment it can be quite noticeable. 

You have to figure out what is causing the problem before you start spending money. Is the hum in both channels. Are your phono cables shielded? What happens when you move the ground wire? 

First, you should determine the type of hum you are dealing with. There are two basic types: 120Hz buzz, typically caused by ground loops, and 60Hz hum, typically a result of poor shielding, cable problems, or close proximity to strong magnetic fields.  When the turntable is selected, does the hum increase with volume.  If not, you have a ground loop problem, if yes, more than likely poor shielding etc.