2dB tolerance is what the gal said on the phone to me.
The serial numbers on my repair pair are 214808 (L) and 214843 (R).
If you have an old pair of Etys just send them in and upgrade which will cost you about $200. I had an old pair of the HF 3 that didn't work and sent them in.
I received a new pair of the ER-4PT for $200. I think this is a good deal. They sound amazing and much better than the HF3 0r 5.