I've got Herron Audio line stage preamp and phono preamp. He's got polarity rocker switches on the backs of both; he recommends trying them each way but says a lot of people can't hear any difference, and I could not.
He's also got an 'Invert' on the remote and on the line stage preamp, and that does have the effect of switching the wires on the back of one speaker, and makes a huge difference. When in phase, there's a well-defined center image - when out of phase, there is not and the bass (and most everything else) is weaker.