A/B testing, with the pre-requisite level match (very important to better than 0.5dB!), makes you focus mostly on different frequency response (i.e. 'more bass' or whatever) unless something is very wrong also (distortion).

Today's equipment (sources and amps specially) is mostly freq-response very right, so when people do matched-level A/B they tend to not find differences, specially when done blind (they focus then on the differences and the brain makes all sound more similar!). This made birth to a thining current we all know (?)... but unfortunately (?) not 'all sounds the same'...

Sometimes A/B may help if you know what you are looking for, otherwise you need more time to find all clues ths system(s) may give.

Basement: Linn Sneaky DS, 568.2mm, ARS Sonum Gran Filarmonia, Ramallo Allegro.
Living Room: 602, Linn Akurate DSM, Hypex Nc400, B&W 802D.