So far, Amazon.co.jp have denied any responsibility for this (as the IEMs are outside the returns period), and I have been waiting for a few days for a response from Final.
If I have no warranty cover, do I just have a couple of very expensive baubles for the Christmas tree, or is it likely that a local authorised dealer (e.g. Jaben Shanghai) can organise a paid repair?
I don't think any universal dealers or even manufacturers bother to repair any of their defective products. They usually just give you a fresh replacement since a repair is not worth the hassle. Regardless, you will need your receipt.