The phase switch is likely to be audible only if you have a recording that is completely phase coherent- using 2 or 3 mics in the recording. Any more than that and the phasing information becomes too confused to hear the effect of the switch.
I'm pretty sure we were the first company to offer this feature (1989). Although a minor effect or inaudible for many recordings, the pain in the rear that it can be to reverse the speaker connections in both channels is what prompted us to include the feature. No additional circuitry was needed in our case as the preamp was/is fully balanced-differential so reversing a couple of internal connections is all it takes.
I'm pretty sure we were the first company to offer this feature (1989). Although a minor effect or inaudible for many recordings, the pain in the rear that it can be to reverse the speaker connections in both channels is what prompted us to include the feature. No additional circuitry was needed in our case as the preamp was/is fully balanced-differential so reversing a couple of internal connections is all it takes.