Dug out one of my backup units, which is a late-model Rio Car, possibly a firesale unit, with a serial number in the 4000's range.

Took some work, since it was getting an IDE error at first. After recrimping the cable and resoldering the IDE header just to be sure, it finally booted and I could load the 44/48 test tracks onto it.

It's running software 2.00 with some old version of Hijack, and I can still clearly hear the problem with the 48khz files and no problem with the 44.1khz files.

So that rules out a quirk of my particular old Mk2 hardware as an issue, though we were pretty sure it wasn't that to begin with. It does establish a clear baseline for my next test though:

Next test: Install v3alpha11 onto it and see if that makes a difference.