Pass X150.5 vs. X250.5

I owned the 150.5 in the past and have spent a fair amount of time listening to their amps. I currently own an Aleph for a specific pair of speakers in which it matches well (60/90 watts 8/4 ohm, all class A). IMO, buying a/any Pass Labs amp should be based on your anticipated amount of required power being able to be driven by said amp when it is still functioning in Class A.

My experience with the Pass Labs amps is that their performance when it moves from Class A to A/B changes notably. In AB they are decent amps (not great amps) and become strident in class A/B versus their characteristic sound in pure class A.

As the poster above stated, going from the 150.5 to the 250.5, while delivering more total power, its added power in Class A that is important. In this sense, it is not a big increase (if you look outside of just the percentage increase - 10 to 15 is a 50% increase - but still not much in class A).

Once again, I think you are running out of power in class A mode, not total power. You indicate that when you turn it up, it gets bright and grainy (my words), this is indicative of these Pass amps that transfer out of class A to class A/B, in my experience. The 250.5 isn't giving you much more power in class A. I understand your desire and goals in moving from the Aleph series to these, but I think if you want to stick with Pass, you need to step up to more power in class A, even if it is less overall power in A/B to get the results you want, louder volume without the sound falling apart while achieving the more detailed and attachs that you felt were lacking in the Aleph series (with its more tube-like sound in comparison and its SET based design).

I still wouldn't rule out the room as a contributing factor. Some rooms can handle a certain volume or SPL level and exceeding that level can have consequences that aren't as noticeable at lower listening levels.

Bryon is right IME. Try diffusion on the back wall versus absorption.

Ceiling though is a nice area to address as it too is a first reflection point. But ceilings are a bit more difficult from an aesthetix stand point.