The sub should be getting fed directly off the LFE output of the receiver so it is playing the full LFE range and then you can use the setup of the receiver to tweak the crossover frequency (low pass) which the subwoofer is using. The slope looks reasonable on what you posted, and it really depends on what you are happy with. I would use test material that is known to have good bass response to see if you are happy with it, or if you want a bit more frequency response from the units you have.
I honestly never considered using anything to further tweak the audio which will be going to my bass shakers that I have, but I will keep that in mind as potential consideration.