Of course there is no such thing as zero latency just as there is no such thing as an instantaneous action. Everything takes time. Its just a question of how much. Even pure analog circuits operate at 80% the speed of light. Of course this is undetectable to the ear. A mic placed within an inch of an electric guitars string can capture the sound with 1/12 of a millisecond of latency. This is 36 times faster than the KPA's 3ms typical latency snd therefore a suitable reference.
My suggestion for measurement is technically sound. Try it and let's get some real measurements.
bd