I can ABX it to <1% uncertainty on my laptop using grado SR60s. It's all in the transients. Lossless is more punchy, accurate, AAC more sloppy and well… lazy. But AAC really gave me a run for my money. I haven't done ABX for >200kbs yet, but I might not be able to tell. If I weren't used to the FLAC I would have a hard time spotting AAC from a lossless mediocre recording. Well, I guess I don't have lead ears.