BTW:
To all the nay-sayers, please take a look at the OP’s data. Notice how the original measurements showed output at 20 Hz very close to the output at 1 kHz. This is despite his speakers being rated to -3dB at 40 Hz.
This is a very similar situation to the one I wrote about in my blog and why I keep saying measurement is important, and we may be adding subs for the wrong reasons. Let the OP clean up his bass first, and then decide, based on his own needs and values if a sub is worth the trouble.
In terms of what this SHOULD Be, I am going to go with Floyd Toole and Anthem's ARC room gain recommendations. Below 100Hz there should be a shelf of around +3 to +6 db. Basically this is "salt to taste" when it comes to bass levels, and follows my overall process:
1 - Measure
2 - Clean up
3 - Re-shape