I was lucky enough to be in my local Audio t (now Sevenoaks) when the guys from Naim were in setting up an NDX , 202, 200 , Hicap, napsc with the ovator S600's. Sat around had a coffee, looked at the coffin sized boxes, did a lot of drooling while the Naim guys were tweaking the speakers in the room. Then listened to Tracey Chapman's self titled album (on CD, through the shops dem CDX2, as they couldsnt get the prototype NDX to work!!).
Bearing in mind that other than the speakers, CDP and listening room I have the same amplification so know that quite well, when the ovatars were up and running I was gobsmacked. I know I dont like the sound of the CDX2 (hence why I have a REGA saturn) as its a bit too exact / analytical for my tastes, so removing that from the equation, the speakers just delivered, everything in spades. Depth, texture, bassssss but very controlled. Tracey's voice was incredible natural and it was one of the shivers down my spine moments in a good way.
I remember saying to the Naim guys not to take this the wrong way but the speakers simply dissapeared, they may as well not have been there and you could have been listening live. That was with a moderate (in the Naim Heirarchy) level of Pre/Power.
Are they worth 6K, yes, if your system is at least the level above and you have a LARGE listening room. Have not managed to dem the S400's yet.