Listening is, I agree, the only way to judge any hifi component. However, A/B testing is susceptible to human foibles unless it is double blind. This is why all scientific tests are carried out double blind. You can't trust human perceptions!
One of the key issues is "expectation bias". If you expect an outcome you are more likely to report that outcome. None of us can avoid this. We have surely all experienced a time when you never see some event then it appears in the news and you see it all the time. This is why medical trials are double blind. If a patient knows they are getting a new medication they are more likely to report that it works! Even then, in most trials, about 25% of those given a placebo, usually a sugar pill, report that it improves symptoms.
Years ago during a similar debate on here the then editor of What Hi FI said they would never take part in a double blind test. I wonder why! 🙂
I was a complete sceptic on DAC's until November last year and erred towards the "it's just 1's & 0's" consensus but because I had no real experience of trying many wasn't as outspoken to go into threads about them, telling people they were wrong.
I kept hearing people say there is a difference so eventually relented and thought I've got to find out for myself ... if so many people believe it there's no smoke without fire.
I also didn't WANT to spend any more money on HiFi equipment but when I 1st plugged in the Chord I could instantly hear the difference in soundstage width I DIDN'T NEED A BLIND TEST TO HEAR IT ....... seriously, you'd have a real hearing issue not to notice it.
I also didn't want to move from the WiiM / Amazon Eco system to Bluesound as I their App and lack of compatibility with Amazon was a complete PITA for me and also meant I was forced to movge to Tidal.
My "BIAS" you refer to was:
Stay with WiiM streamers (I own seven of them). - no expense
Not buy a Bluesound Nano - expense
Not but a new DAC - expense
I have completely gone against what my bias, preference and convenience was .... you don't do that without reason.
As I sugeested in one of my earlier posts , how about you share with us your experience and which DAC's you've tested extensively and by that I don't mean a demo in a music shop .... I mean having a home trial for at least 3 days and able to do instant A/B testing for at least 4 / 5 hours and which you ere comparing them aganst.
Please also share how you volume matched them, how you were able to instantly switch from one to another and what your music source is and how you were able to get it across 2 units at the same time.