If the room is small(I thin it is 13/14'?)...with no large openings to other areas...room gain will start in the mid 40s. Adding (in theory)12dB/octave. In the real world...figure 9dB/octave max. But that is still 9dB by 22hz and about 15dB by 16hz.
If you can measure the subwoofer outside(50-60ft from the nearest reflective boundary) this will give you an easy way to find out what the room is adding to the low freqs.
Remember, just because the SPL meter is reading *X* SPL output using say a 16hz test tone...that no way implies the output of the subwoofer is only 16hz. Chances seem good the subwoofer in question could be producing a lot of the 2nd harmonic(32hz) and maybe even a lot of the 3rd and 4ths.(48,64hz). The harmonics (harmonic distortion)will show up on the SPL meter as output too.
Even order harmonics tend to sound *harmonius* to the human ear...so you could have a subwoofer producing 50% THD and if it is based mostly in the even orders(2nd,4th,6th,ect)...it won't sound too bad. Odd orders tend to sound quite harsh though. Odds are usually produced when the amp clips or the driver exceeds linear excursion capabilities. Harmonics are often to blame when folks contend they can *localize* bass <100hz. It isn't the real bass they are locating...is it the harmonics which are much higher freqs(and not attenuated by any crossovers).
If you have the time and a PC nearby...goto
www.soundtechnology.com and DL one of the spectra analyzers...you can then see the THD as you perform your tests. I have a bit of experience with PC based RTAs so if you have any questions...fire away.
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