That shield still must be connected at one end or the other in order to actually stop RFI from getting into the signal. It will connect with the 'ground' wire and as such still is considered part of the signal circuit.

If the shield is not connected on either end it can help somewhat, but the effects are extremely minimal if it is not grounded to something.

You are correct though, I misinterpreted his request, he was curious about balanced output amplifiers.

I haven't looked into them much, but from a brief overview it looks like they are very similar to a bridged amplifier, where you have two amplifiers working in tandem, one with reverse the polarity of the other, to drive the speakers. I would imagine this would have much of the same effects as the balanced line level does.