As mentioned above, the treble tube might be clotted on one side. However, there might be another explanation:
I have compared a brand new pair of SA6 against my well burn-in pair, and it is very obvious the they sounds quite different as mines sounds much smoother and balance on the mid-range. The brand new pair sounds more fuller as the mid is more forward. Jason has said it before that SA6 requires at least 50hrs of burn-in to reach its final sound signature and I agree. In your case, it could be the drivers are unevenly burn-in, resulted in mismatching freq. resp. in both transducers.
I have compared a brand new pair of SA6 against my well burn-in pair, and it is very obvious the they sounds quite different as mines sounds much smoother and balance on the mid-range. The brand new pair sounds more fuller as the mid is more forward. Jason has said it before that SA6 requires at least 50hrs of burn-in to reach its final sound signature and I agree. In your case, it could be the drivers are unevenly burn-in, resulted in mismatching freq. resp. in both transducers.