The Radio Shack is just not designed to accurately measure low frequencies whether it be noise or sinewave. It cannot respond near as well as other slms. However, come to think of it, you could be having some cone breakup on the sub, that may give you that problem. Also, the radio shack is better than nothing I agree, but its absolute accuracy can vary by as much as 3dB from low frequency to higher frequencies (ie 1kHz) I've had to calibrate these things and what I really wish is that dealers would spend money for a type 1 SLM with octave analyzer, and really dial in a customer's system. As you nodoubt know Don, even an exact C weighted curve is going to change the reading when you are in a passband mode with a subwoofer.
