no... when you subtract a negative number, that's the same as adding right? example: 1 - (-1) = 2. not 0.
so, with + on one conductor, and - on the other... you get:
hot - cold = 2 * hot (since cold = -hot)
get it?
so, the interference is cancelled out, but the signal is the sum of the absolute value of the voltage on each conductor.
but again, don't sweat it if your stuff doesn't have balanced outs... balanced lines have little advantage for most home audio setups where equipment is close together, and there aren't many cable being used. in a pro studio, this is different, cause many studios have more than 1 mile of cable being used in close proximity to each other... in that case, balanced lines can help a lot.
you can however reduce hum problems using the same type of cable balanced cables use, but in an unbalanced configuration:
to do this, connect the one conductor to the "hot" on both ends. then connect the other conductor to the ground on both ends. also, connect the shield to only one side, usually the side that's going to be connected to an output. this configuration is what is mistakingly called a "half-balanced" or "semi-balanced" cable. technically, it has nothing to do with balancing... but anyway... it does help with hum somewhat.