Bob,
You can indeed do this. To start, you turn the speaker on one side and work from underneath the speaker. If you have the soundanchor stands mounted to the speaker...they must be removed including the base that attaches to the 801 itself. A large allen wrench is used here.
From there, you unscrew the bottom wood cover to expose the XO board. If you have the soundanchors already, this cover may already be off. At this point, you follow the instructions supplied by NC, and as I mentioned above.
George Short is a great guy, and you will be able to call him for any info needed during the set up if you need to. He will surely be most happy to help you by phone.
As Tab110s mentioned, if you dont already have the Soundanchor stands for these..by all means you NEED them to place this speaker off the floor. It changes the sound so much for the better, that its like night and day difference. The addition of the external crossover will add refinement and dynamics to an already nice sounding speaker.
Ken