Compensation chart

The Radio Shack sound meter is not totally "linear" in its accuracy down there. All such meters are off by the same amount depending on the frequency however, which is good news! So that while its sensitivity below 20Hz is not what it is at 50Hz, there is a simple way to compensate for this deficiency. It's called a compensation chart. Using one is very simple, and a pencil and paper is all you need.

Simply look to the below frequencies and add the required decibels (dBs) at the frequency in question. So if your system seems to be measuring 70dBs with a 20Hz tone, then really it's putting out 77.5dBs. This is a VERY substantial difference, roughly TWICE the acoustical energy, requiring many times the amplifier to achieve. 
 

NOTE: Using sine waves at high SPLs can easily damage any subwoofer given their extreme stress on woofer voice coils.  Use only a few seconds for any given tone during measurements, allow a minute or so in between frequencies, and keep actual SPL's low, in the 75-85dB range.  Contact SVS if you have questions about room response measurements or proper use of sine waves in this process.

So find out what your subwoofer(s) are REALLY doing, and add the following numbers as appropriate to your SPL meter readings:

At 10Hz add 20dBs to the meter's readings and at ...

12Hz add 16.5dB
16Hz add 11.5dB
20Hz add 7.5dB
25Hz add 5dB
31.5Hz add 3dB
40Hz add 2.5dB
50Hz add 1.5dB
63Hz add 1.5dB
80Hz add 1.5dB
100Hz add 2dB
125Hz add .5dB