johnbasham,
Usually the test discs (Avia, Sound and Vision) have a frequency sweep test from the high frequencies down to the low.
During this sweep, you should register a relatively even dB reading on the SPL meter unless there is a 'hole' in some range.
In such case, you will hear the sound dropout, the meter will register it and using the frequency sweep tests (and specific frequency tones) you should be able to narrow down where that hole exists.
Of course you should keep in mind the frequencies should also naturally trail off around/just past the driver crossover points.
