I find generic ABX tests not great, personally, because I generally don’t know what to be listening for. However, with songs I’ve listened to lossless my whole life, it’s much easier to spot encoding failures - an intuitive “wait, that cymbal crash sounded different” or “that multi-instrument harmonic should be cleaner/dirtier.”

That being said, 320Kbps AAC encoded by Core Audio I’ve found to be pretty much transparent with anything I’ve thrown at it. Anything less than that (256Kbps AAC, 320Kbps MP3, etc) I can ABX sometimes, as long as I’m familiar with the source material, and usually only with quality headphones. Although no streaming services provide that, so I’m stuck with ALAC through Apple Music for streaming (which is more convenient than my old solution, which was transcoding and transferring to an iPod ~20k songs selected from ~90k in my library based on a variety of rules than never gave me the song I’m looking for). And really, ~900Kbps lossless is pretty easy to justify these days with 5G data speeds and generally much higher data transfer limits.

The other downside to storing losing encodings these days is the fact that almost everyone uses Bluetooth for their listening, which is an additional lossy encoding. While 256Kbps AAC/320Kbps MP3 might be transparent in some cases, when it’s re-encoded it very rarely is (in my experience)