To determine the compression is going on with the mains in relationship to the subs. Put a displacement laser on the sub and you can find the max spl for the system because usually the sub distortion happens much sooner that the mains. One exception is under sized mid-bass drivers that are asked to go to low.
Typical max SPL is given at 1 meter....... Need to take into account your listening distance and then the room gain in your listening environment.
