......Class A sound can be addictive however and many fans are okay with the tradeoffs....
I somewhat agree but, yeah, there are many who would sacrifice a class-A sound since getting a bigger class-A amp to satisfy their needs would be very expensive.
But the beauty of the Pass AB models like the x250.5 and x350.5 are that they deliver 90% of the sound you will be listening to at most sound levels in class A but can supply the extra grunt when you really need it.
the "grunt" you are talking about is, I assume, the current delivery capability of the power amp? If yes, this ability is set by (or limited by) the power transformer. For example: take the XA30.5 power amp. I'm guessing it uses a secondary 20VAC transformer. You can get a 400VA/20VAC transformer, dual secondaries. Such a transformer would be capable of 10A/ch. As a 2nd example, take the X250.5 power amp. I'm assuming this power uses a secondary 45VAC transformer. You can get a 1000VA/45VAC dual secondary transformer. Such a transformer is capable of supplying 11.1A/ch.
So, the current capability of both the XA30.5 & X250.5 amps is very similar. Yet the X250.5 is recommended by Pass Labs for speakers such as B&W. Why is that?
The answer lies in the DC rail voltage of the power amp section. For the XA30.5, the DC rail will be 29VDC. For the X250.5 the DC rail voltage will be 63V - more than 2X the rail voltage of the XA30.5!
So, when you have a speaker like B&W which has some really crazy impedance & phase responses over the 20Hz-20KHz region esp. impedances where the resistance is gets high, the XA30.5 power amp will simply run out of voltage headroom. As an example, say, the speaker resistance at a particular freq is 4 Ohms. If the max rail voltage is 29VDC, if the power amp supplies 7.25A, the voltage on the output will be 29V & the output stage of the power amp will be saturated i.e. it will start clipping. So, the XA30.5 is capable of delivery 10A max but you cannot even make use of that capability 'cos the power amp output stage saturates. OTOH, if you are using a X250.5 & the same 7.25A is supplied, the 29VDC generated in the output stage is much lower than the max 63VDC rail voltage & the X250.5 doesn't even break a sweat.
THAT is why Pass Labs recommends the X250.5 over the XA30.5 for many/most speakers. It's quite clearly stated on the Pass Labs website in the answer to the question "What is the difference between the XA.5 and the X.5 amplifiers?"
The XA30.5 would be great for a speaker with a benign impedance curve i.e. an impedance curve which remained relatively flat over the 20Hz-20KHz region. There are a few speakers that do this & for better for worse they all happen to be 1st-order x-over time-coherent speakers.
Crazy impedance curves are due to high order x-over circuits that need to compensate for sub-standard speaker drivers which change their impedance widely over their (frequency) band of operation.
Abrew, I think I understand that amp-speaker selection is not only sensitivity & nominal impedance ratings & that Qts does play a role in this selection. The Qts information is very difficult to get a hold off. However, measured resistance & phase plots are much more accessible to the public (thanks to Stereophile) & reading these impedance plots one can get a pretty good idea of what sort of power amp one might need to get superior sonics.