[quote author="Ethan"]...what happens when the balanced positive and negative signals are ideally identical, and there is no "diff" to amp?[/quote]
If the positive and negative signals are identical, then this signal is "common" to both of them.
Hopefully if this ever happens then it will be a small amount of noise or interference which is the same on each line. These will then cancel-out because when the differential amp goes to amplify, it only amplifies the difference - if the noise is the same on the (+) and (-) line then they will not be amplified. The pure microphone or line signal is usually opposite on each line (hence (+) and (-) ) and is therefore amplified.
Now you can look at "common mode rejection ratio". It is the ratio which shows how good the differential amp is at rejecting this noise / interference.