It probably does blend better at 80 -- that's not at all uncommon. In fact, it's rarely truly adding more. Reflections will cancel out other reflections, while adding to others, and so on; there will be both constructive and destructive interference. The best way to do the crossover point is so that you can't tell when it switches. Try everything, and guess what, it all depends on what you like to listen to. Don't do audio to please others, but neglect yourself. You paid for your stereo so enjoy it however you'd like. That said, technically it could be bad, as generally the smaller driver that can do a frequency at the right volume will be more accurate because of its reduced mass, but do what sounds right to you. What you should do, however, is go to Radio Shack and purchase a Digital SPL meter and fix it to your listening position (a camera tripod works wonders here) and then just make the transition as smooth as you possibly can (messing with phase dial / switch is good here).
Good Luck
-Will
