HiFiMan Sundara headphones, focusrite scarlett 2i2 interface. Although, it turns out the Scarlett is set to 48KHz anyway, and I can't seem to change it easily under linux. Not that it seems to matter for my ears, lol.
EDIT: Did some more ABX testing with a CD-quality track that I'm much more familiar with ('Introduction' from the Mirrors Edge soundtrack, which has been my go-to for comparing audio gear for the last decade). I could sometimes distinguish 128k mp3 this time, though interestingly, I got it consistently wrong rather than right. For some reason the compressed version seems to be my preference. Dropping to 96k mp3, I got it right 100% of the time - though only because there was a very noticeable difference in the stereo positioning of the first sound, rather than a difference in the quality of the sound. I think if it were mono I would still be unable to tell.
EDIT: Did some more ABX testing with a CD-quality track that I'm much more familiar with ('Introduction' from the Mirrors Edge soundtrack, which has been my go-to for comparing audio gear for the last decade). I could sometimes distinguish 128k mp3 this time, though interestingly, I got it consistently wrong rather than right. For some reason the compressed version seems to be my preference. Dropping to 96k mp3, I got it right 100% of the time - though only because there was a very noticeable difference in the stereo positioning of the first sound, rather than a difference in the quality of the sound. I think if it were mono I would still be unable to tell.