Pass X150.5 vs. X250.5

Where the meter sits is completely arbitrary, some sit at 12, some sit at 11, others wherever. What matters is if it moves. If it sits and does not move, then there is no leaving Class A. If it bounces all over the place then you need more power. The X250.5 is definitely a jump in power, but as many have pointed out it is doubtful that is your problem, it is most likely something else in the chain. Is the X250.5 a better sounding amp, to me yes, but if you have fatigue it is probably something else.

What is your source?

Acurus makes a very important point and I agree 100%.
What is the location of your volume control knob?
Does your meter move from class A to AB?

Yes, Pass meters only reflect Class A to Class A/B (unlike McIntosh meters). if it is moving a lot, you probably are underpowering your speaker.

A Vandersteen 3A? In that size room? You might be underpowering, but I would think one is listening too loud if that is the case with an X150.5 - no? - I mean, that is a lot of power for that speaker especially a 87db /nominal 6ohms (I would think)