Hi ddirado,
Since you are running the subwoofer at higher than normal levels, the readings you will get from your Radio Shack SPL meter on the C-weighting scale won't accurately correlate with your sense of subjective loudness. The meter averages sound-pressure levels across the frequency range, and the bass will cause unusually high readings, although it may not seem all that loud. (Our ears are not very sensitive to deep bass, which is why the levels have to be cranked up before we hear them.) It's more a function of the SPL levels in the midrange, which is where most of the audio content is centered, that give you the sense of "very loud", etc. When loudspeaker sensitivity is measured, the curve for the noise signal is weighted to de-emphasize the bass frequencies.
THX cinemas are supposed to set the average levels in the audience at 85 dB SPL, but I've found that these standards are routinely (and grossly) exceeded. I fled a free screening at New York's Ziegfeld theater of the last Star Wars movie. It was unbearably loud even 2/3 of the way back in a very large movie theater. I didn't have a meter with me, but at the time I guessed that average levels were well into the 95-dB+ range, with extremely high peaks.
Regards,
