Difference between polarity and phase w/sub

I am not familiar with your equipment but have usually found polarity switches to operate in the preamp section to alter the polarity (as described by Swampwalker) for the output of the preamp. Rowland, among others, used to use this to deal with amps or other devices which invert phase. I also don't know why manufacturers produce products that invert phase, but there may be a good reason I am unaware of. Subwoofers often have a phase switch (0= in-phase, 180= out-of-phase). This adjustment is the same reversal of polarity (+ and -) as seen on preamps but used strictly at the subwoofer to integrate with the main speakers as Sidssp outlined. It is no different than if you just switched your cables at the sub. Proper setting for this will be a function of sub placement relative to the main speakers and Sidssp's method is the way to determine what works in your application. You want the two bass sources to be as in phase as possible and this will become evident as the amplitude increases around the crossover point. If you can't hear which setting is louder at that frequency area, use a test CD, and if necessary, a Radio Shack analog DB meter. I recommend the test CD by Rives which compensates for the inaccuracies in the meter and may help you place your woofer for the flattest response in your room. Room placement is extremely critical for subs, despite what some advertisers claim. Random placement and reliance strictly on volume and phase adjustments rarely leads to satisfactory bass integration. I hope this helps.