I guess the main thing is if the app can measure consistently across your system.
Right. That was their main use, to balance the volume in the various channels of home systems. But getting a decent level of accuracy in loudness measurements, including the ability to calibrate these meters, only comes at a much higher price.
Volume balancing with SPL meters has largely been replaced by built-in software such as Audyssey.
I highly doubt it would be trustworthy for level matching subs though considering the roll of of a phone mic that’s designed for human voice.
Level matching subs is the least of the many problems facing an owner. Sub woofer placement to minimize subtractive losses due to out-of-phase room reflections is a much bigger problem. Often placement alone doesn't solve the problem. Room correction software attempts to deal with that, but success can still be elusive. I guess it's still a work in progress.
