Does it have something to do with how the Radio Shack meter processes the sub test tone?
Farewell Voom. Farewell Directv. Farewell Bell Express Vu. Farewell Dish.
Had 6 dishes, now none.
Back to Comcast. Farewell Comcast, now back to Dish 120+HD. Farewell Dish.
Returned to Comcast 06/2012.
Back in the early days, there were just HD demo loops, no programming. Now HD 3D, amazing!
There are some frequencies where the RS SPL is deficient. I remember printing out the posted list and using it about 10 years ago. There was a list of corrections for the analog and the digital meters. The lists were obviously for calibration purposes and not for general dB levels.
I wouldn't worry so much about an SPL measurement for the sub, but be more concerned with a full bandwidth response without the bass being too boomy... Set the other loudspeakers @ a balanced SPL level, set the subwoofer level on the box itself to about 50-60% rotation. Next starting with the Subwoofer's level set @ 0dB in the AVR, you should only have to make minor tweaks..
Also physical positioning of the subwoofer within the listening room can greatly affect the bass, distance from the wall or being in a corner delivering the most bass.
Keep in mind, that the sub level can/will vary greatly with various source materials with music or cinema tracks.. Plus personal taste of the listener..
Just my $0.02..