When I have done A/B blind testing I find I can tell the difference immediately the first couple of times, tell which one I prefer out of the two.
However, after a few swaps I struggle, and they start to sound more alike.

Many argue if that is the case then what you buy doesn't matter, however, I argue you should always go with your first impressions as generally we tend to get that right far more often than not.

I do think your brain tries to make everything sound the same when playing the same thing over and over, this becomes fatiguing, doing A/B tests starts to give me a headache, in the same way listening to bright systems gives me a headache, I don't think it is the fact it is lively that gives me this headache more the fact my brain is trying to convert the sound into something smoother that I prefer that does it.

A friend has Cyrus gear with some M&K speakers, and my god that is forward and bright sounding. I find that system too sharp to listen to, and asked him if he found it tiring after a while. I was amazed when he said he didn't find it all that bright.
What amazed me more was when listening at mine I asked what he thought of the 5200s compared to his set up, and he replied "Sounds very similar."
[censored]!!!!? shocked

They are about as far removed as you can get with two systems, the Meridian is really smooth, they have always rolled the top end off their kit to give it that liquidy sound.
I thought about this for a while and then decided it must be us, our hearing, for him to feel they sounded the same.
I asked him to play an online frequency hearing test and although I could hear up to 22kHz no problem, not only did he not hear anything above 17kHz but he also struggle with lower frequencies on the test too, he was a OK with 10, 11, 13, 14, 15 and could just hear 16kHz but strangely he could hardly hear 12khz.

So, maybe some people genuinely can't hear big differences when doing A/B testing, and because they can't hear differences they can't understand why others can?
However, it is more that that, the differences a good bit of kit and one that connects with you can often be the most subtle of all, sometimes you put a bit of kit in and everything you play on it sounds right. There was nothing wrong with the other bit of kit, but this one just nails it every time. And I think those are the bits that can get missed when doing A/B testing.

No Darling, I've had it months!