When mixing subs, one needs to make sure two things: 1) bass extension phase response characteristic needs to be close to each other (for instance, 45 degrees phase shift difference or less), and 2) set up the gain proprotional to their max output so no sub can overload too early.
Here is the phase response of FV18. The blue line display the phase shift from +180 degrees (top) to -180degrees (bottom). Each division is 45 degrees. THe noise below 10hz is due to signal being too weak (too much attenuation).
1) How do I read the above plot?
A: In a plot like above which is a basspass, find the 0 phase point and start calculating phase shift from there. Keep this in mind, phase is like a clock. Once you complete a resolution, it starts from 0 phase again and two, -180 degrees is same as 180 degrees in terms of phase angle. In the above plot, the zero phase is at 55hz. Above that, the phase decreases, which is typical for a low pass filter. Below 55hz, the phase increases which is typical for a high pass filter. At 20hz, the plot rotates from 180 degrees to -180 degrees. So at 12hz, the plot shows 0 degree, but in reality, it is -360 degrees, not 0 degree, and not 360 degrees. This is very important when you consider the big picture of phase coherence. In addition, in the midbass range, the more phase it shifts to the negative side (going downward trend), the more delay on time domain which means it is as if the sub has been placed further back. This information is helpful when one tries to set up multiple subs with slightly different upperbass phase shift or distance to MLP.
2) Can I use placement to compensate for low end phase difference?
A: No. The reason is any physical distance based approach can amplify phase difference at mid-bass and upper bass range. For instance, 20hz has a wavelength of 55 ft. Half wavelength is 27ft. The same 27ft distance that results in 180 degress (which is half wavelength, 27) at 10hz will actually cause 4 times of phase shift which is 180x4=720, meaning in the above plot, it resolves twice when we go from left to right. It causes more phase issue.