For a fairly recent A/B experience, I have a Maverick Audio TubeMagic D2 DAC that has parallel solid state and tube buffer outputs. I had listened to both regularly and formed a strong enough preference for the solid state output that I didn't even both having interconnects hooked up to the tube buffer output. In the process of upgrading, I purchased a Schiit Modi Multibit DAC and found it quite enjoyable. After several weeks of listening I had the idea to compare it to the tube buffer output on my MA DAC and oddly enough found them indistinguishable to my ear. I can only attribute temporary preference for the Schiit Modi Multibit DAC to confirmation bias/placebo. I also have a Pioneer Elite SACD player that was indistinguishable from the solid state output on my MA DAC. I found some information on upgrading the MA DAC, so upgraded the tube (Western Electric 396A) and three op amps (Sparkos Labs) and now strongly prefer the tube buffer output and find it far superior to my SACD player. Now that I have an upgraded interconnect on my DAC I don't even bother using the SACD player anymore as it's simply disappointing in a way that makes any A/B comparison a waste of time.
If time and money were no object, I have have two identical pieces of everything so that I could always compare A/B with only a single piece of gear changing. I'd confirm that I couldn't tell any difference with the dual setup and then swap a single item and compare and see if I could distinguish a difference. It would be fun, but totally impractical.
I read on a forum somewhere a great quote that if an audiophile can see a difference they will hear a difference and I think there's some truth to that though I don't discount that with enough time and effort some people will be able to distinguish things that are far more subtle than most of us can.