For dialing in phase, what's important is not the absolute SPL magnitude but rather the relative magnitude. Therefore the accuracy of the meter and linearity of the LFR and sub don't matter. What matters is repeatability. Meter repeatability isn't an issue and as long as the levels are kept low, driver coil compression isn't an issue either.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated