Updates on my experiment,
At 75hrs of music, still able to differentiate my pairs in a blind test, although getting harder sonically.
I them put them on 24 hrs of pink noise this weekend, and I'm still able to differentiate, however, it's predominantly based on noise / noise floor at the moment. The sound signature is about the same, however, the new ones still have a lot more noise floor. I'd also almost swear that the older ones have > sensitivity / volume for the same setting.
I'll keep them going to see if I finally get to the point where I can't tell them apart.