> This is largely because MP3 is a crappy, old format. Try high-bitrate Opus on most samples and you're a lot less likely to get above-chance results on an ABX test, pretty much ever, at high bitrates.

This is such a key point: MP3 has known weaknesses with certain common sound profiles (e.g. sharp transitions such as a cymbal or snare drum) which aren't really fixable. For me the threshold was AAC 256kpbs where I stopped noticing artifacting.