That's all you have to do. Just be sure your amp doesn't reverse phase as well (if it does, you don't have to do anything).
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There is something called "absolute phase". If a single speaker is wired out of phase, it's initial excursion is back, not forward. You can feel this by putting your finger on the speaker cone and feel if it pushes out or in when, for instance, a bass drum kicks. All the speakers can be in phase with each other but the whole system can be 180 degrees out of phase in reference to the original recording. Even a turntable cartrige can be wired out of phase and screw everything up from the start.
So begin with a single speaker and make sure it's not 180 degrees out of phase and reference the other speakers to it. Also some speakers can be mismarked at the terminals. A simple test is to take a 9 volt transister battery and, using a test lead with aligater clips, connect the positive of the battery to the positive of the speaker. With your finger on the diaphragm, feel the excursion as you touch the negative aligater clip to the negative lead on the speaker. If it is correctly wired the diaphragm will push out when you touch the terminal with the aligater clip. This will tell you if positive is really positive with respect to the internal speaker wiring. To be sure, test all the speakers in our system. Finally, rocket science explained!
Dc2, there was a previous discussion of this. I had always thought as you do that the contact with direct current should be an outward push of the driver. Now I am less certain. Consider the microphone and the initial impulse of a horn. The microphone moves backward. If this electronic impulse is a positive pulse the driver would be pushed outward, but if the impulse is negative, it would pull the diaphram backward. All would depend on the microphone being hooked up out of phase. I wonder if there is any consistency to this.
Also, it is improbable that a preamp with phono will maintain absolute polarity throughout or between the phono and non-phono inputs.
I have a little unit for inverting polarity on phono but never use it. Most often, however, you can hear polarity differences on LPs. With purist digital players I no longer have the easy choice of polarity on digital. My old parafeed Exemplar preamp could easily have had a polarity switch by just reversing the output side of the transformer.