I, too, was initially disappointed with my N804s, but after about 300 hours, and an upgrade or two in upstream components, I'm now pretty happy (which is about the best that a neurotic audiophile can hope for). I found that a high-current amp was critical for bass, and that a good digital front-end (Wadia) made a huge difference in all areas.
As to your specific issue, you may want to experiment by making a partial "hood" for the tweeters out of felt padding. Absorbing the tweeters' side and rear emissions close to the source is more effective and cheaper than treating the walls. It looks stupid, but it's a really great experiment, and you may decide that being able to place the speakers close to a wall with the hoods is the most practical solution. Also, since they're front-ported, the bass is still pretty tight near a wall. Finally, don't underestimate the amount of energy that the Nautilus tweeter sends out of the hole at its rear. Using just a bit of felt to absorb that may be all the difference that you need.
Happy listening, and let us know what happens.