Mark, If you wire a speaker out of phase the signal still comes from the same speaker post on your amp because only one of them is driven, the other is connected directly to GROUND inside your amp.

A cartridge is floating, yes. But a cartridge is a symmetrical generator with +/- coils that provide exact mirror images of each other. A speaker is not a generator it is the other type of transducer and is only driven on one side, because, wait for it, the other side is connected directly to GROUND inside your amp.

If you doubt what I'm saying you can do one of two things, open your amp and look at the lack of output stage to drive the black speaker post- see- GROUND. Or you could ask someone who knows more about electronics than you who you are willing to believe.

You cannot stick a little passive box on speaker wire and make it do common mode noise rejection- it is impossible, not unlikely, improbable or anything similar, just plain old impossible.

:-)