upgrade advice

I think you have made an excellent choice for your upgrade.

Update - I have upgrade to a McIntosh C2200 pre and MC252 amp. So far it has been a true upgrade; soundstage has widened, low-level lsiteining has improved, dynamics have exploded.

I also have Nautilus 804. Pass Labs amps are the best sounding amps I've heard in my system. I've tried X150.5 and X3. Both sounds marvelous. X series are much more affortable. X .5 series sound better than X series, but is more expensive. Get X .5 series if you can. However, the difference in the sound quality between X and X .5 series is smaller than the improvement coming from non-Pass to Pass amps, so X series may be a better value for you.

My listening room is 15 x 20 x 10H. Power was never a problem for me in the room, with the smallest amp being 75wpc Jeff Rowland model 2. You have a larger room, but still even the smallest X series amp are rated for 150wpc and probably is sufficient to drive 804 in your room. If you opt to get larger Pass amp, you do get more pure class A power in low levels.

Rotel gear is pretty good for the price but it can be bettered. With the B&w I think you already layed the foundation to what can & will be an excellent sounding system. A friend of mine seemed to like the Krell sound with these speakers. But to get further advice from others it would be good to know if you like a warm or more accurate sound? Are you thinking of inserting a tube preamp in the mix?