It's known that the only difference between well-prepared files of different bit depths is the noise floor. Basically, in any PCM audio file, you can expect noise of about 1 ULP. So in a 16-bit file you get a noise floor of -90 dB, and in an 8-bit file it's at -42 dB. This is the ONLY DIFFERENCE.
People who are here listening for distortion artifacts aren't going to find any, and they're going to be surprised. That's because you can always choose to introduce noise instead of distortion artifacts when you do the conversion. The noise is actually added on purpose, it's "dithering noise", and it's not there to mask the distortion, but to entirely replace it.
The math works like this. Take a high-resolution input, say, 16-bit. You're converting it to 8-bit. If you just convert, you'll introduce a distortion signal of about 1ULP. However, if you introduce 1ULP of uncorrelated rectangular noise, the distortion signal is now also uncorrelated noise, which is less objectionable than correlated noise. It's also harder to hear.
Then, how do you tell the difference between 16-bit and 8-bit audio files? You can only do it if there's a quiet part somewhere in there. As soon as there's a quiet part, you can hear a "hissing" sound in the 8-bit file at -42 dB. If you're listening to the middle of a rock song like this, the hissing sound is going to be buried in the mix. Heck, a typical guitar amp will already be putting out uncorrelated noise.