Critical subwoofer tip

I recently purchased a sub to augment the bottom end of my Spatial M3 open baffle speakers. Due to domestic constraints, one sub is all I will have in the living room. 
The sub is front firing, and placed about 18” from the rear of the Spatial. If you follow me here, I was concerned about the back wave (open baffle) cancelling out the sub’s forward wave. I made all my adjustments- xover freq, volume, Q, listening mode (Eq).
Things sounded very good- then I flipped the phase to 180 degrees.
This caused things to snap further into focus- I imagine the back wave of the OB speaker was interacting with the sub at some frequency, causing cancellation perhaps?
I mention this because it may very well be that some sub/ speaker interactions will not be affected by the switching of phase- sealed box speakers maybe, front ported, etc., on and on. Maybe rear ports with subs behind the speaker are affected differently.
This could explain why some people hear no difference when flipping phase- I have a separate home theater system, and for the life of me I never heard a difference when doing so. In this 2.1 channel system, it is readily apparent to me.
As I write, I also realize that the 2.1 system uses a sealed sub- the HT subs were ported. Maybe porting changes things too. Just more to consider...

More than a few post confuse polarity with phase. If nothing else, maybe this discussion will point this out.

180 degree phase shift LOOKS like reverses polarity, when graphed. However, 180 degrees phase shifted is actually 1 cycle delayed.

Inverted polarity will substantially reduce LF. (Subs out of polarity with the main speakers) 

Now, after the polarity is correct, we can optimize the phase. 

So, we must optimize sub: level, crossover, polarity and phase. 

I think it is safe to say that I am one of the confused ones. If we ignore changes in amplitude over time (impulses) and just stick with a sustained tone at a given frequency, how are two wave fronts 180 degrees out of phase not the same as two with the polarities reversed?
If I had a sub with speaker level inputs and no phase control, I would expect swapping the speaker leads to produce the same effect as flipping the phase 180 degrees.
BTW, I am enjoying these posts and appreciate the OP for starting it and everyone who has contributed. It has caused me to think more about things I "knew", or should have known, and how poorly I have considered some of those things in practice.

One of my subs remote equalization presets is set for reverse polarity.
Any recording with a prominent Kick drum will reveal proper polarity.
By simply switching the assigned preset button the Kick will sound noticeably more or less forward.
This only changes the polarity within the subwoofer system.

Making the manual switch at my preamplifier does a complete switch.

Since polarity can change from track to track on some more highly produced recordings made at different locations it would be most useful if every modern preamp was equipped with a remote control polarity switch since its more discernible at the listening position.  

Any recording with a prominent Kick drum will reveal proper polarity.

The kick is a poor arbiter. One might have a preference, but unless one knows the material first hand, it's just a preference.

Since polarity can change from track to track on some more highly produced recordings made at different locations it would be most useful if every modern preamp was equipped with a remote control polarity switch since its more discernible at the listening position

Polarity can vary from instrument to instrument in one track!

Back in the day, the east coast & Europe was mostly pin 2 +ve, while the left coast was pin 3. Some studios varied in both locations. Microphones could be either depending on their history.

Pop overdubs were often done all over town. Sometimes on both coasts or overseas. Not all engineers checked polarity. Hence the drums could be +phase, lead vocals -, percussion +, backgrounds ?, etc.

Some more twisted engineer types carried their own mics, pre's, EQ, amps, speakers & cables. 0:) That way you could get the room without worrying about polarity.