Hi All,
I'm wondering if my Pass Labs X150.8 amp would be sufficient to "comfortably" drive the new B&W 803 D3 speakers, which I'm considering. I'm not looking for ear-splitting, rock-concert level sound, though I do listen loud.

The room is only 14' x 14' with a cathedral ceiling. For that matter, any opinions as to whether the speakers would even overwhelm a room of this size? Any opinions much appreciated! Thanks very much.
I had a Pass Labs X150.5 prior which was rated at 150 watts 8ohms, 300 watts @ 4 ohms. I believe the X150.8 is rated exactly the same. 

Your speakers have a sensitivity rating of 90db, correct?  Sheesh, you could power those with a 50 watt amp if you don’t play very loudly.  

My listening area ceiling slopes from 9 feet to 17 feet at the apex - a difficult space to get “right”. 
The speakers I rotated through during my Pass Labs stage were Magnepans - allegedly “power hungry” speakers - and Thiel, models CS3.5 and CS3.6 - also allegedly “power hungry” models. 
(The CS3.6 more so than the CS3.5)

What you need to consider is the current the X150.8 is capable of when being driven at the levels you prefer, and how low the speakers dip when pushed hard. The Thiel CS3.6 are rated @4ohm,  but that dips considerably when driven “hard.”   I believe it’s sufficient, but you may want confirmation from Pass Labs to be certain.  I think what you have is quite sufficient. 

Mind you, I’m not a tech kind of guy. Frankly, I don’t know a mosfet from a misfit,  but I read the manuals, ask questions here on the ‘Gon, and use it all.  

PS: those read like incredibly good speakers. 
I drive my 804 d3s with the Pass Labs xa25 with no problems. You should have plenty of power with 300 watts at 4 ohms.
Thanks very much for the reassuring responses!