Try reading some of the other subwoofer based threads that have taken place recently. You might gain some insight from those. Sean
To properly cross over your sub to your mains, you must check the specs of the "crossover" to see what slope is used(look for a graph in the owner's manual) and whether it just crosses out the mains, or handles the sub as well.(If it does both, then the sub will plug into "sub" outputs on the receiver.) "Setting the rolloff" as you refer to, sounds like a switchable setting on the receiver to determine the slope that each speaker(main and sub) taper off in volume, around the center of the crossover frequency(90Hz?). Shallow slopes(6db) allow the sub to play well into the lower midrange, and steep slopes(24db) will cut it off much lower and more quickly. If your sub is not crossover controlled by your receiver, then you must match the slope of your sub's low pass crossover(filter) to the slope of your receiver's high pass crossover(filter).At the same frequency(90Hz?). This ensures smoother transition between mains and sub. Generally, you set them both the same:ie. set sub to 12db and mains to 12db. Imagine a hill with the crossover frequency at the top and the sub rolls off on one side and the mains roll off on the other side.Read the user manuals of both the receiver and the sub.