Speaker hiss from power amp

I have a Mark Levinson 532H amp that is connected to a Levinson 390S CD processor only (no preamp).  Recently, I switched from single-ended to balanced interconnects (Cardas Neutral Reference) because it's a pretty long run (~12 feet) between the components.  The 532H has switches on the back for selecting balanced or unbalanced interconnects.  The switches had always been set for the RCA single ended inputs, so I just noticed that when the switches are set to balanced, there is a very slight hiss coming from my tweeters.  When unbalanced are selected, the hiss is substantially reduced, nearly but not quite silent.  Both speakers have an equal amount of hiss.  In balanced mode, the hiss is only audible when my ear is 3 or 4 inches from the tweeters so it doesn't in any way detract from my overall musical experience.  I should note that the hiss is evident whether or not anything is actually connected to the amp (other than the speakers, obviously), and there's no difference with gain when I have the 390s connected.  Also, the addition of a power conditioner has made no difference at all.  Two other things: there is a very slight hum coming from the amp (again, only audible if I'm very close to the amp) and the A/C power from my outlets seems pretty inadequate (probably less than 5 amps according to the Parasound).  Appreciate any ideas on why the hiss increases when the switches are set to balanced inputs and how it could possibly be eliminated.