When crossing over the mains to the sub, you should really let the Room decide which freq to crossover, and I would be sure not to cross over lower then 60hz .
By saying "let the room decide" I mean that you should be testing the freq response with a spl meter with a sweep of test tones and graphing the output, if you have a big dip in output where you are blending the mains to the sub, that may not be the best place to cross over. You want the graph to be as flat as possible where your crossing over, if its not flat your room may be presenting a problem to use that setting. Experiment with a number of different settings to make sure you choose the best for your room....
Go
here to download a test tone CD and read that page... As you can see in the graph on that page, they have a very nasty looking graph... What you are looking for is a nice smooth response much like this...
http://i34.photobucket.com/albums/d109/Warpdrv/HT-Great Room/HTPics014.jpg