Below a certain frequency in most common household rooms the room pressurizes rather than merely reinforcing from the proximity effect. Below that frequency the bass is acoustically increased by as much as 24dB SPL.
I Just made some measurements of a subwoofer starting outside for a pseudo-anechoic response at 2 meters and then in a small room (12 x 11.25 x 9ft) at various locations. I posted a long description in my subwoofer placement thread.
But, using this chart I could make assumptions about "in room extension"
So, by industry standards, the subwoofers "anechoic" response could be listed as 40 - 115Hz, +/-3dB with a bass extension of 34Hz (-10dB).
However, the "in-room performance" could be 40 - 120Hz, +/-3dB with an in-room bass extension of 30Hz (-10dB).