I would assume that you have used an SPL meter to set the levels? It also depends on whether you are talking movies or music. I would not go entirely by the specs. If you don't have an RTA (real time analyzer) then the SPL meter with a test disc with test tones (e.g. download at
http://mdf1.tripod.com/test-tones.html or a disc such as
http://www.delosmus.com/cgi/cart/ite...17501074135006) should be able to tell you where you about where mains drop off in the room (every room is different). The ideal for music vs. home theater might be different. Bass frequencies sum, so for a music only system one would normally set the crossover frequency for the sub at a point that is below where the mains drop off in the room. It would depend on the crossover and the properties of the crossover (it may be different using what is in your receiver vs. the sub crossover controls).
If using what is in the receiver you would also want to make sure that the sub crossover on the sub is not set at a point below what you are using in the receiver and therefore leaving a hole in the reponse. So in summary, measure what your speakers play in the room and then measure with a test disc and try different combinations to see what gives you the smoothest response in the base area.