Summation check
Separate curves don't tell you how sub and speakers play together. The Summation check does: two sweeps, one for each position of the phase switch.
Why it matters
Around the crossover, sub and speakers play the same frequencies. If their waves reach the listening position in phase they add up; if they arrive out of phase they partly cancel, and the bass loses body right there.
The sub's phase switch changes this relationship, but the difference doesn't show in curves measured separately. They have to play together.
How to do it
From the results, open the Summation check. Keep volume, crossover and the iPhone's position as in the previous measurements: the two takes are compared with each other.
Sub and speakers together
Turn on subwoofer and speakers, confirm the current position of the phase switch and tap Start combined sweep.
Now the opposite polarity
Move the sub's phase switch to the other position and tap Start inverted sweep. If your sub has a phase knob instead of a switch, use 0° and 180°.
Slope compares the sum in the band around the crossover: the one set on the sub, if you entered it, otherwise the suggested one.
Reading the outcome
- Better with: normal (0°) or inverted (180°). With that position the bass adds up better around the crossover. Leave the switch there.
- The two positions are equivalent. At this spot polarity makes no difference: pick the one you prefer by ear.
- Polarity does not improve the blend. Flipping the switch isn't enough. Try another crossover or another position for the sub, then redo the check.
The check tells you which position makes sub and speakers add up better at your listening position, not how correct your system is in absolute terms.