Piedpiper, I agree about the degradation. I had a Millennium preamp made by Siltech, with a remote switch to invert. I noticed that the best sound was to invert the speakerwires at the speaker but not on the preamp. It turned out to be an inverting preamp and did use another stage to allow inverting. I was hearing the loss in using the inverted setting. As you say some designs could easily provide this. All line stages with parallel outputs even if they just use a transformer could provide it without an additional stage. Parafeed designs could also.
Would it not be great were recording engineers to chose one polarity and stick to it? All you would have to do would be to decide which you preferred with your speaker connections and stick too it.