> Can you really quantify nonlinearities from a sin sweep of any sort? Not that I'd expect a decent set of headphones to have material nonlinearities unless seriously overdriven.

You can using a properly structured log sine sweep but not with a linear sine sweep. This is why the paper mentions "Log sine sweeps rather than linear sine sweeps were employed to allow verification that non-linear distortion components were virtually absent."

You can read up more on why here: https://pdfs.semanticscholar.org/abc8/3f1297e5c033b8322f3d90...