There is a widespread school of thought that I subscribe to that it is better to run the main speakers full range rather than use a crossover. If you do use one it is better to insert a passive attenuation one in the signal going to the main speaker amp. While rolling off the main speakers will allow you to play them louder the crossover will degrade the sound. This does not bother the HT fans who value dynamic range and think special effects are more critical than music. I would look on the DIY audio sites for advice on constructing a passive attenuation network. They will also have info on who could build one for you.
Sub Question
I tried to do a search, but could not drill down on this. Anyway, I have never owned a sub. I never really felt the need. However, I had a friend tell me that a sub does more than just enhance bass output and perhaps bass quality. He went on to say that having a sub takes pressure off the driver thus making the speaker do less work. Because of this, the speakers will sound better or have better clarity. Thoughts?