Human beings are not very good at auditory memory, one has to do an immediate comparison under the right conditions and then our ears reveal interesting things. A simple A/B switch is cheap to put together and shouldn't cost a fortune to buy complete. Doing this should be eye opening, highly recommended, but you must carefully match the volume. If you mismatch the volume of both amps, one will sound better, which can explain why some people come to certain conclusions. Flipping the switch back and forth is an amazing experience.
could you suggest an A/B switch with decent quality and low price? do I need to buy two 3.5 cable too?