How confident are you that the 16-bit recordings you buy have been properly dithered? At a minimum, 24-bits means one less thing to get screwed up.
There's a lot to admire about the survey, but I'd like to see participants train on 10-bit, and 12 through 16-bit files compared the 24-bit source ; with training complete, weed out those who can't tell 12/13-bits from 24-bits; THEN take the 16-bit vs 24-bit test. I'm unconvinced the survey respondents know what to listen for, and it can vary by recording anyway. The results from trained listeners might still be null, but I'd find them more meaningful.
that is a good idea, so people can start looking at the real bit depth they hear. from my own experience, I can be bothered by noises around -70db, not on all songs and I'm known to be overly sensitive and obsessed with any kind of noise or hiss. but -80db I really can't hear it with music at the same time. I need to crank up the volume too much for my own good and pay attention when the music is on a very calm part. so 13/14bit would probably be enough for me for the rest of my life.
about the dithering, the test was done with a not great dithering setting on purpose it seems. but even without dithering at all, would people notice? to me dithering is more of an insurance than a need.