So then what setting would be flat?
That depends. If you took them outside or put them in a huge room then it would be flat with no bass management engaged. However, as soon as you put a large powerful speaker like that into a room you get room gain. It is typically in the 35Hz to 100Hz region.
How many db does each setting change output?
I have not measured how many db per change. Measuring accuracy in that low of a frequency range outside an anechoic chamber is spotty at best.
You said it allows for "attenuation", so does that mean it only cuts the amount of output from flat(no boost)?
Yes, attenuation means to turn it down. There is no boosting anything with a passive network.