I don't know if this is right place to put this topic... but I was just reading the guides on using a SPL meter to calibrate the sub, if I don't have an inline equalizer, how else am I able to adjust the different frequencies to remove spikes?

Also, what are the benefits of flattening out the frequency response curve?

I have HSU VTF3 mk2 and MBM-12.

I don't think you can do it thru the receiver. You would have to use an eq like the BFD. Repositioning the sub or adding room treatments can also improve the sound of your subs.