Pass Amps Class A operation

I would add to the previous comments that perhaps the most significant factor is the dynamic range of the music you are listening to. The fact that much popular music is highly compressed dynamically (i.e., at a fairly constant volume) will make relatively few watts sound subjectively quite loud. Jazz and some classical chamber music will also often have relatively limited dynamic range. The worst case would be well recorded (i.e., relatively uncompressed) classical symphonic music. That would cause you to turn the volume control up to get reasonable volume on the soft passages, which would result in very large instantaneous power demand on orchestral peaks, bass drum whacks, etc.

Btw, it looks like your speakers have a rated sensitivity of 87db/1W/1m (representing 2.83 volts into their 8 ohm nominal impedance), just a bit greater than that of the Maggies which Dne referred to (85db/2.83V/1m, representing 2 watts into their 4 ohm impedance).

Regards,
-- Al

1 watt =87db
2 watts = 90db
4 watts = 93db
8 watts = 96db
16 watts = 99db
THX is 110db, ultra THX is 112db

A couple of things to keep in mind:
1-there's no crossover in the amp, just over a certain load, it starts switching out.
2-you could be playing in class A all day long at 5 watts which is fairly loud. However, if there is a dynamic peak the amp will automatically fall into class b and then back again. So, it should be seamless when switching.

There are really only 2 ways to know: The amp will lose some of its warmness & the amp will run much cooler. In class A, the circuits are running at peak all of the time. They have to dissipate the power somehow if you don't need the volume so it's given off in heat.

On my X250's meter, with significant movement indicates the amp is moving out of class A. It’s somewhere about 12 o’clock that the amp is into A-B operation and the first 10% is class A watts.

I was thinking almost exactly along the same lines at Elevick. Continuing on those lines then:
I'd like to make a few mods to the values posted by Elevick - you need to reduce the SPL at the listening chair by atleast 9dB.
The listening chair is not going to be at 1m away from the speaker; it's probably more like 3m (9.9ft). The SPL drops 6dB for every doubling of distance. From my *assumption* that your listening chair is at 3m, you need to subtract 6dB+3dB = 9dB. Additionally, there'll be absorption from the rug/carpet, drapes, furniture, etc. So, subtract another 2-3dB.
That brings us to 87dB/1W/1m - 9dB - 3dB = 75dB SPL at your listening chair when the 150.5 is outputting 1W.
2W = 78dB
4W = 81dB
8W = 84dB
16W = 87dB
Now, since the music is in stereo, you need to add 3dB to the above numbers. So, if the 150.5 is outputting 4W, you'll be getting 84dB SPL at your listening chair. That's pretty loud - you'll need to raise you voice to talk. At 8W, you'll be getting 87dB SPL. At this point you cannot have a normal conversation in the room.
Of course, this does not include dynamics peaks for which the amp has no class-A capacity (probably by design) & it'll need to go into class-AB & then switch back (just like Elevick wrote).
So, overall you have most of your listening in class-A barring dynamic peaks but w/ this amp you do not have any class-A wattage to spare for dynamics. All your dynamics will be class-AB.
I think Pass' X350 has 75W in class-A - in this amp, you could have some reserve wattage for dynamics in class-A mode.

The power to SPL calculation is pretty much aligned with engineering understanding. Yet reality does not seem to support this kind of calculation. For example if we can all agree that 2W is all needed to produce 78 dB at 3m away, then adding extra cushion by a factor of 5 even (to bring the power requirement to 10W) is not bad an engineering cushion, really, to account for dynamic peak or other Murphy's law situations. Yet in reality, would we buy a 10W/channel amplifier to drive anything? Even a factor of 10, bringing the wattage to 20W/channel is a no-no.

I like the idea of using the needle movement on Pass equipment to tell when the amp. moves out of class A. My speakers has a 4-ohm impedance, meaning 120W/channel drawn from the XA60.5. The needle vibrates ever so slightly around 100 dB when my ears were about to burst because I accidentally set the volume too high. The vibration is very light, as the needle likes to lean back to its static position around 12 o-clock when the dynamic peak is gone.