Most all of the above, but if you are running sub woofers to augument the bass of 2 way speakers, not to relieve the mid-range drivers of some of the load created by doing the bass, try running the main speakers full range and rolling off the subs as low as possible. A SPL meter can help finding the ideal cross over point as well as positioning them to take advantage of, or minimize room nodes/nulls caused by placement of the speakers, subs and mains, as well as the listening position.
Just a WAG, but if your speakers are -3db at 45hz, and your subs have a 6db per octave cross over slope, I would agree with Timlub and try setting the cross over at 40hz or lower. But consider the location of the sub will have a lot of influence on your decisions, its not just the cross over.
Just a WAG, but if your speakers are -3db at 45hz, and your subs have a 6db per octave cross over slope, I would agree with Timlub and try setting the cross over at 40hz or lower. But consider the location of the sub will have a lot of influence on your decisions, its not just the cross over.