High pass and low pass filters

I am in the process of intergrating a sub to my 2 channel system using a crossover. If I set the high pass to 80hz should I also set the low pass to 80hz? Seems like the logical choice to me, but I'd like to make sure.
Of course -- that is where you start.
Later, in fine tuning the system, you may be led to play a bit with the high pass.
You see, the room interacts with the speakers (obviously) and getting the bass right is tricky...
It might be a good place to start but likely not end there. Many variables at play here: room; sub position; steepness of frequency roll off or cut off in the filters; etc. Chances are, with everything set at 80Hz, everything will play somewhat above and below that setting. For example, one time I had an 80Hz low-pass filter on my mains and ended up setting my subs at 55Hz for smoothest transition in my room. For me it was faster and easier to dial in my subs with an SPL meter and sub setup CD than by ear.
An Audyssey/SVS sub controller or Velodyne SMS-1 sub controller is the easiest way that I know of to integrate a sub seamlessly. These devices also add digital room correction in the bass region which is IMHO a fantastic benefit. They're not inexpensive - the fully automated Audyssey/SVS is about $800 IIRC, while the Velo SMS (which still takes some work to achieve best results) is about half that price.

Good Luck