Pass X vs XA amps sound quality in class A range

Thanks for your replies, I have the X250 driving a pair of B&W N802, room size is 32ft by 42ft by 20ft height, at their widest, a fairly large room and open to one side to dining room. Driving the speakers w/o the current meter moving (in class A?), it sounds mighty fine but when driven harder, there is a gradual hardening of the sound and loss of air. I am curious as my upgrade path should lead to either the X600.5 or the XA160.5. If they both sound similarly excellent at 50W/ch in class A, I would go with the X600.5 just to safe for those parties that I tend to crank it up.

Much thanks again for all comments, I think you have answered my questions, that they sound different even at low power levels.

Thanks Pete, you may have saved me some troubles to just go straight to XA.5 class! Although my room is huge, my listening habit is low to mid in volume, to protect my hearing. In fact, with the lowly X250, I rarely see the bias needle move at all ever. But when I throw a party, it's different!

Another question for you all then: When the XA series exceed their rated outputs, do they cross over into class AB or clippings occur right away?

Thanks again, you folks are first class!