@caloyzki: Here's a good one I have on my Samsung Galaxy: Sound Meter
It's calibratable and once it's done with the AVR's internal test tones at 75 dB, the sub of course reads a 65(ish) dB. I think it's pretty much enough for relative measurement of multiple subs.
Which one do you use?
I think the main problem with using a phone app SPL meter is the mic in the phone. I'd imagine it was optimised for speech frequencies and so it may be fairly useless for bass frequencies. IDK for a fact - just seems a reasonable assumption. It may still be good enough for relative (as opposed to absolute) measurements though as you suggest.