Pretty much, yes.
That is it. For this to work, you must have a device that takes the sum of two signals of opposite polarities, i.e. a differential pair:
Actually, common-mode rejection has nothing to do with signal polarities, or indeed signals at all. Consider: Balanced interfaces reject common-mode noise even in the absence of any signal.
I can't see how noise that is conducted equally to both terminals of a moving coil speaker (whether you are using a bridged amplifier or a ground referenced amplifier doesn't make any difference here) results in the elimination of said noise.
Because a moving coil speaker is a differential input.
Think for a moment.
A dynamic loudspeaker only works because of current flowing through its voicecoil.
Apply equal voltages of the same polarity to each of its terminals.
How do you manage to get any current to flow?
1V - 1V = 0V.
Since I = E/R, 0V/R = 0.
I have done home audio installs. It is a guideline to never allow a speaker line to parallel a power line less than a foot away (preferably further). Failure to follow this can result in 60Hz conducted into the speaker line. The speaker will not, under any circumstances, reject this 60Hz interference.
Well, first, the typical multiway loudspeaker isn't exactly balanced. The series crossover elements are used only on one side, which creates an imbalance in the source impedance, and as the source impedance becomes imbalanced, common-mode rejection diminishes.
But beyond that, provided that the 60Hz interference is received equally in both lines, i.e. common-mode, the driver would indeed reject it.
The definition of balanced signals is well and precisely defined in the industry. Changing the meaning of those terms is counter-productive.
I've not changed any meanings if any terms.
Bridged amplifiers are not balanced.
Technically they are, as "balanced" refers to a balance of impedances.
What they are not is differential, which means they cannot reject common-mode noise. Common-mode noise at the input of a bridged amp is simply amplified and passed along to its output.
Headphones (with the possible exception of electrostatics) are not balanced.
Yes, they are. Their voicecoils are effectively symmetrical.
Neither will reject common mode noise.
Bridged amplifiers will not because they're not differential. Headphones will as they are differential.