That's correct. When activated, the polarity switch simply inverts the polarity of the signal. This converts positive going waveforms into negative going ones, and vice versa. If the speaker is reproducing a positive pressure pulse, then inverting the polarity will turn it into a negative pressure pulse.
In your situation, there should be no need to activate the polarity button. Your subwoofer is placed very close to both main speakers, so any path length phase differences between the subwoofer and mains are very small at the 80 Hz crossover frequency. At this frequency, the wavelength of sound is 345/80 = 4.31 metres (14.1 feet). To cause 180° of phase shift between the woofer and subwoofer output, the subwoofer would need to be sited 4.31/2 = 2.16 metres.
Below is a VituixCAD simulation of the response of the JBL LSR310S subwoofer and the LSR308P MkII loudspeakers, with zero offset between the subwoofer driver and the 8" driver. 4th-order Linkwitz-Riley high-pass and low-pass filters have been assumed, with 80 Hz cut-off filters. This shows good blending of the responses.
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In your situation, the subwoofer is placed quite close to the main speakers, so the phase difference between the separate outputs is quite small. It will likely affect the blending of the low frequencies to only a small extent. This can be compensated for simply by adjusting the level of the subwoofer relative to the main speakers.
For example, if we add in a 0.500 metre separation between the woofer and the subwoofer, we get the following response, which is only slightly changed from the previous one. The small sensitivity to such changes is due to the use of the Linkwitz-Riley filter topology, with 4th-order filters. I checked with JBL Tech Support, and was informed that, to the best of their knowledge, that 4th-order LR filters are used in high-pass and low-pass sections of the LSR310S.
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