BW Nautilus 805, better to have them ported or sealed

Good question.
B&W has the largest research facility of ANY speaker company in the world. I wouldn't fool with the basics of their design unless you really have a lot of knowledge of design.
That said, what you want to do can easily be tested. after you get the subwoofer of your choice, set it up and see what YOU like best, with the port open or closed. I would be willing to bet that you will end up leaving the speaker as B&W designed...open.

Before I would make any major change, I would want to know that the person or persons designing the change, really know what they are doing, and hopefully be able to hear the change before making any big alterations on my equipment. And, I did do just that with the B&W800's. Dan D'Agostino of Krell designed an active crossover with bass alignment and designed some minor alterations with the crossovers and wiring for this speaker. I followed his advice and ended up with the single largest improvement I ever made to my system, bar none.
Enjoy,
Richard

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.

Some B&W speakers with ports come with foam plugs you can insert in the port to adjust the bass output. I have a pair of CDM-1 and P5 and they both came with foam plugs for the ports. The foam is not a seal, just a reduction of the air flow.

The ports in the B&W 805's are designed to prevent bass bloat, while still maximizing bass in a small speaker. That's what those dimples in the port are for. I use my N805's with a REL sub and do not have any overlapping of bass frequencies whatsoever. Would love to have a second sub though, it would stabilize room modes. I guess I'll have to wait until I get a bigger place to live in before acccomodating a second sub.