I certainly don't question your experience, I just think the mechanisms at play are not located at the driver but in our own firmware so to say
As you say, you expect each Grado to sound great -- that's the bias I mentioned. Then it sounds different from what you expected. Your brain registers that difference and calculates an offset to "great", applying that offset the next time you hear that same headphone. I think this effect is more pronounced with Grados as they sound quite different from most other headphones...
An example form my own recent experience: as you might have read in this thread, I've had several run-ins with the GS3000x. My initial expectation was guided by the GH2 and statements from several reviews as well as the German distributor. Then I had a defective model -- twice. Not an exactly nice first experience per se, plus it sounded *quite* different from what I expected. My local shop was nice and took them back, adding them to their demo pairs, which was OK for them since they did not have a GS3000x on demo.
Over the next few weeks (months actually) I tried them again and again, ultimately coming to terms and even liking them: now I spend most evenings with them, listening for sometimes several hours. Interesting thing was though: after I finally took them with me again, the shop told me that literally *no one* used them in the time they were with them. So they had maybe 1h more on them than when I heard them for the first time.
A bit different with my PS2000e, which I bought used (and well used, if I look at them...). My expectations were different, already knowing the GS3000x, plus they sound a bit more like my beloved GH2, so that was easier.
I also didn't like the GH2 that much when I first heard them, despite them being a well-used (but cared for) demo set. Now I love them -- OK, in that case it only took about 3 sessions to know I wanted to own them.
Another model I've tried multiple times but still can't get to like is the RS1-x. Out of interest I'll try again though
Anyway, my point is not that important overall. In the end the joy of music is what counts -- however one reaches that.