That requires infinite bandwidth (the start of the rising input signal).
Something real world amps do not have so all amps will do this in some severity.
In an FFT (and FR) you will thus see the higher harmonics slope downwards and square-waves being rounded off.
And again.... this thread is about things that cannot be measured. Your example is not something that cannot be measured and a mystery.
And the imbalance of the volume potentiometer can be measured precisely.
But, but, but the imbalance caused by the asymmetry of the listening room, by its furniture which is just as much, will in any case have much more importance on hearing than an imbalance of 0.1 dB and a fortiori by 1dB of the volume potentiometer... ... in the days when there was a balance adjustment on integrated amplifiers and monophonic tracks to adjust the balance from the listening point, it was rare for this adjustment to be at 12 o'clock. . even if we did this empirically with our ears, but on a neutral, inexpressive signal, it worked very well...
In short, all these things are measurable...