B&W - THE Nautilus.

I heard these speakers once.  They were -extremely- clean sounding, but a lot of that could have been from the "Class D" amps by Classe that were driving them.  They require 6 individual amplifier channels to drive them and use an active crossover module to provide the amplifiers with the proper signals.

A couple things to keep in mind.  These are not huge speakers.  The picture may make them seem bigger than they actually are.  Also, since the woofer is basically in a sealed shell, they do not have a lower bass that a normal vented speaker would have.  They come across just a bit light sounding because of this.

That being said, they did sound really good and are absolutely beautiful constructions.

To echo some of the sentiment from the gentleman above, I too have had the pleasure of listening to these and it sounds like it may have been the same traveling demo system. I heard them with the then debuted Classe Sigma series. There was the Sigma SSP going to the Nautilus x-over and then out to 4 of the Sigma AMP2 2 channel amps (each speaker requires 4ch of amplification). The sound was absolutely stunning and I could have listened for a long time. Easily one of the top 3 best sounding speakers I've heard in my 12 years in this industry. That being said and as the same with the 800 series of that time, the midbass and woofer require a good amount of power for them to perform. My B&W rep at the time warned us before hand and while setting them up that the Sigma amps weren't enough power to run them at 100%. He suggested 2 pairs of their CA-M600s for the woofer and midbass and another pair of CA-M300s for the midrange and tweeter. Having dealt with the 800 series back then I can attest to them being power hungry vs. the new D2 and now D3 speakers, but when you do add big power the sound is amazing. 

Their set and placement can be tricky because the speaker can be tilted in every way and the bases are heavy to adjust toe. Once dialed in they are just pure magic! I can't speak about used pricing and it'll be tough to gauge since they don't come up for sale in the used market hardly. 

Just make sure you have plenty of power, get them set up right and sit back and relax!