If the conditions such as PA5 output, usage time and room temperature are the same, there should be no difference in PA5 temperature.I was was using it today with my E50 at max and using the knob to adjust volume today. No louder really than normal and my room temp is the same. It was warmer to the touch. Just kinda warm with my Pre controlling volume to kinda hot today. I use it for hours while working from home.
The only other variable is that I was using Filter 2 which does roll off later than 1 and 3. Just 44.1khz on the DAC though.
But you have come across a mystery.
A controlled blind test or measurement is needed to determine if it is a fact or a misunderstanding.
However, when it comes to temperature, a thermometer is clearly better than a finger.
Prepare a multimeter like this link. Must be compatible with thermocouples.
Play music and adjust the volume of E50 and PA5 to the usual position. Then do not touch the volume.
Turn off the power of PA5.
Remove the speaker cable from the PA5.
Attach the multimeter test leads to the speaker terminal of PA5.
Set the multimeter to the ACV range.
Set the volume of youtube and OS to 100%.
Turn on the PA5, play a test tone and note the voltage.
As long as the voltage is the same, the temperature can be compared regardless of the combination of the volume positions of E50 and PA5.
The thermocouple measures the temperature of PA5. Fix it on the surface where the temperature is likely to be the highest.
When making comparative measurements, fix them in the same position each time.
Record the temperature one hour after playback. Record the room temperature at the same time.
Even if the room temperature changes, comparisons are possible to some extent.
In any case, please arrange the conditions as much as possible.