I think the key word there is developed. yours was faulty from the start. any shop that has a faulty product from new should swap as a matter of course once it has been used however unless it is an obvious fault they do have the right to send it away. i.e. a "waterproof jacket" the leaks after 2 months, it may well be faulty but how do they know. it needs to be tested. If some one brought a jacket back to one of my shops but was going away for the week walking we would lend them a jacket whist it was away for testing, we didn't need to do that but we always looked after our customers.
If you do go back and they refuse deal with it reasonably then take details, who have you spoke to when, where, what did they say, keep anote of phone calls and what they say. That way if you can't come to an agreement you take it to the small claims court and you have all the details to hand.
I would ask for a refund now, for one of the pairs, take a note of his response and tell him you will be asking his head office for clarification. call his HQ and ask for the Ops manager(my old job)