Reversing Polarity -- Voodoo or Easy Tweak?

If you reverse the leads on only one speaker, you will be out of phase, and your imaging will be wrong. This can be easily checked with any available test disc. It's pretty easy to hear. A single vocal will not have a centered image, but will instead be split across the soundstage.

Absolute phase is something different. To reverse absolute phase, you reverse the +/- leads on both speakers. Absolute phase is more difficult to hear, but it's easy to experiment. BTW, you can reverse leads at the amp instead of the speakers with the same end result.

Some manufacturers (Bat for example) allow you to easily swap absolute phase with the remote & others (Hagerman for example) give you manual switches to play with.
My take from playing with a BAT integrated I owned was that on most recordings keeping absolute phase 'correct' sounded subtly better and on one or two recordings (The Blues Brothers being a prime example)swapping absolute phase made a nice improvement.
I think a lot has to do with how well the recording studio does at keeping track of absolute phase in the mixing & recording processes.
FWIW, a dealer I know has a classical album that was recorded out of phase and you have to swap wires on one speaker only to get it to sound good, & I suspect that on many recordings some of the mixing is 'right' and some of the rest is 'wrong'.

Jeff_jones...Speaking of what I will call "relative" phase (one speaker with respect to the other) the "proper" phase is not always clear. Out of phase signal has a diffuse and directionless quality, and that might be what the recording engineer intended. In a matrix multichannel setup out of phase signal localizes to the rear channels, so reversing one set of speaker wires just swaps front and back speakers. When played back on a two-channel system a source that is intended to be well defined center front (typically a soloist) is a monaural signal (same in both channels). All recordings have some degree of out-of-phase signal, often nothing but ambience. However, there are some exceptions. One good example is the recording by Buffy Sainte-Marie, "The Angel" where out-of-phase signal is used with stunning effect.

I do not suggest that anyone deliberately connect stereo speakers out of phase, but a signal being out of phase is not inherently "wrong", and can be used to great effect when a recording is mastered.