If nothing else having two SPL meters give one the option of eliminating the meter as the culprit when a problem like your rapid bass rolloff crops up under test.
I have hoped for the last 15 years that my SPL meter would turn out to be perfectly flat when tested against another. No other response ever made any sense when my sub shows a +30dB house curve to 15Hz at the listening position! I had exactly the same problem with my previous DIY bandpass boxes in an even larger listening space.
I'm tempted by one of the new soft-look analogue meters myself.
