You just can't go changing a ported enclosure to a sealed enclosure, speaker design simply doesn't work that way. Drivers, cabinets, and cabinet designs are mated to each other. The entire cabinet would need to be redesigned if the same driver was going to be used in a sealed enclosure. Sealed enclosures require larger volumes for the same frequency response. Just because a speaker is ported, doesn't mean that the bass is boosted. By porting a speaker, a lower frequency can be achieved in a smaller cabinet. If the bass output is artificially boosted by a port and the speaker doesn't provide a flat response, that is simply a design flaw of the speaker. The key to integrating a subwoofer would be to find a smooth transition between the low frequency response of the N805 and adjusting the X-over on the sub accordingly.
Additionally, the bass alignment filter that Krell produced was for the Matrix 800 series of speaker and not designed for the Nautilus lineup. The bass alignment filter is not an active crossover in the regular sense of the word. It simply enhances the low frequency signal to correlate to the Matrix 800 series of speakers to provide a lower frequency response. The filter that comes with the standard Matrix 800 speakers is not very good and the Krell is definitely superior.
Additionally, the bass alignment filter that Krell produced was for the Matrix 800 series of speaker and not designed for the Nautilus lineup. The bass alignment filter is not an active crossover in the regular sense of the word. It simply enhances the low frequency signal to correlate to the Matrix 800 series of speakers to provide a lower frequency response. The filter that comes with the standard Matrix 800 speakers is not very good and the Krell is definitely superior.