In 1978-79 I worked at an audio dealer who had an "electronic" switching system, so one could A/B not only speakers, but amps, preamps, CD players, etc.
One day I setup a customer to A/B a McIntosh MC2205 (solid state amp) and a Luxman LRS (tube amp) and he spent a LOT of time switching back and forth, seeing the lights on the display change from "A" to "B". After over an hour doing his comparisons, he told me at length how he perceived the differences between the amps and purchased the Luxman.
A few weeks later our repair tech notified us that he was replacing the circuit board in the unit, as it switched between the "A" and "B" lamps, but was only playing the amp connected to the "A" input, and that this was the case from the time the unit was installed.
One day I setup a customer to A/B a McIntosh MC2205 (solid state amp) and a Luxman LRS (tube amp) and he spent a LOT of time switching back and forth, seeing the lights on the display change from "A" to "B". After over an hour doing his comparisons, he told me at length how he perceived the differences between the amps and purchased the Luxman.
A few weeks later our repair tech notified us that he was replacing the circuit board in the unit, as it switched between the "A" and "B" lamps, but was only playing the amp connected to the "A" input, and that this was the case from the time the unit was installed.