Sure thing, so the low bass adjust is a shelf filter that boosts or attenuates all the way from 0 Hz to 80 Hz. The height of the shelf is up to 6 dB, and you move the entire shelf by 1 dB with each adjustment that you make on the sub. The curve of the shelf back down to the baseline starts at about 30 Hz. Once you get to 50 Hz you're at the half way point down the curve with respect to 30 Hz and once you reach 80 Hz you are almost all the way back to the baseline.
As for the parametric EQ, each change in the level (+1 or -1) will change the peak of the effected frequency band by 1.5 dB.
As for the Q factor, it refers to the width of a range of frequencies. The Q factor is inversely proportional to the width, so at a low Q factor a larger range of frequencies are effected in comparison to frequencies that are within a high Q value. So, the Q factor for the "wide" setting is 2.0, for the "normal" setting it's 3.5 and for the "narrow" setting it's 6.0.
If you really want to get into it, the physics behind how Q is calculated can be found here:
As for the peak that you are reading, it is probably a room effect. But you could do some testing to find out for sure. If you place your SPL meter right in front of the sub, an inch or two, then you will minimize peaks from the room. If you can take the sub outside or place it in a very large room that would be even better.
If you still get that peak no matter what, you can use the parametric EQ to minimize it by reducing the 40 Hz band by the same amount of dB as the peak.