You would hook it up between your preamp, and your amp, but then you would be EQ'ing your main speakers. I did not think you wanted to do that. You may lose some resolution to the main speakers.
I was under the impression that you wanted to EQ your sub(s), and not the main speakers. Do you have a second preamp out, not a tape out, but a second preamp out, on your preamp??? If so, take the second preamp out, and run it to the Behringer, then run the Behringer to the sub(s). This way the Behringer is not in the circuit with the main speakers.
Generally, it is the floor to ceiling bounce (peak) that is most likely causing the worse problem. Also the first harmonic of that floor to ceiling bounce will be the next bite on the A$$.
If you have a 8 foot ceiling, the peak will be about 75hz and, it's first harmonic will be at 150hz.
Also, your other room dimension, especially if the room is smallish, can cause a stacking or reinforcing of the peaks. Now I'm getting ahead of myself a bit.
Post your room's dimensions, and lets see what we can see.
Here is a very simple example lets say you have a good size room, 31ft long, 17ft wide, and 8ft high. Our main concern would be with the 8ft dimension.
So lets say we have a 6db peak at 75hz. You would set the unit's FREQUENCY at 75hz (that's the frequency you want to reduce), then set the ATTENUATION at -6db(the amount you want to reduce the peak by), and set the BANDWITH at 3hz (the amount of width you want to treat on both sides of 75hz peak) the 3hz BANDWITH would be + or - 1 and 1/2hz on each side of 75hz. So you would get a 6db reduction from 73.5hz to 76.5hz. See, that's a pretty narrow range, and you will not be effecting the frequency on both sides of 75hz that don't need EQ'ing.
This is a very simplified example and there are more things that can come into play.
I feel that most people blame their subs for being slow or muddy, when it is the floor to ceiling peak that is causing the overhang and the slowness.
My questions to you are (1) do you have a second preamp out on your preamp (2) are you using a subwoofer, and (3) what are your room dimensions???
If you are not using a sub, you may have to consider using the PARC that Rives offers. It's more transparent and designed to work better with your main speaker and not give up any resolution. Using the PARC vs the Behringer for a sub may be a little overkill.
Only problem is, if you have to go for the PARC, it may cost you your next born male. You'll have to work that out with your wife.
If you want to contact me off site that's fine.
Dave