I have one other idea which I think is worth a test - since people report problems with background hiss on other Clip+s and with other files and knowing that I sometimes can hear hiss on my c200v1 during "disk" access (e.g. it's also audible when the original firmware refreshes its database).
If I understand correctly, those high quality files mean more data and the Clip+ doesn't have much RAM so that means quite frequent disk access and I could imagine you hear that. I think it would help to find out if you can relate it to this too, e.g. by watching the disk access indicator in the menu's statusbar (the far right icon) or using a WPS with this element used - or using some files with similar "data density" but a different codec - thinking of WavPack or so.
But yes, making such a file available to a developer which also means testing on different targets would be good too (not sure if you are allowed to post such presumably big files directly to the tracker though).