Fun topic. There's a lot of physiology in perception that isn't well documented or understood. I've sometimes thought of doing a posting on this in sound science because I've not seen it discussed too often but I also haven't done enough homework to see what research exists. Over the years I've...

This is only a guess having not gotten the headphone back, but if you had an intermittent sound quality issue there was probably something underlying it like a broken or damaged trace that could cause part of the driver to have a nonlinear motion, so a new unit shouldn't sound the same.