The agents from AJ told me that sony HK had repaired the down button, for sure, I am still baffled, it stayed just one week in warranty, so I doubt a quick fix, perhaps sony sent them a replacement unit, I will only know when it arrives
Actually a stuck button is a very basic fix. It can be as simple as popping it back in place. The button just rests on a touch pad or plunger type device. All they would have to do is open it up at worst and take the one that is stuck out and pop in a new one if it is damaged.
Given that you have a standard rather than an extended warranty if they can do a simple fix they will instead of giving you a new one given how much a ZX100 sells for. The advantage of the extended warranty for Sony is it covers the replacement cost of another unit so no extra cost to Sony if they send out a replacement. Sending you a replacement under a standard warranty would cost Sony money so they would only send a replacement if your ZX100 can't be fixed.
I spent several hours listening to my replacement A17 and it definitely sounds better than the old one. Clearly Sony made some improvements to the A10 series after the initial release. The fit between the top and bottom of the case on the new one is tighter (closer together) than my old one. I can remember on my old one if I pressed the top and bottom of the case together there was some give, on the new one there is no give at all. The whole unit just seems more solidly built. As I mentioned already the screen seems to be better to.
I think Sony gave me a basically new unit as a replacement because my old one was from the first batch sold in North America. It simply wasn't made as well as the later produced units.
I guess it pays to wait a few months after a DAP is released before buying one in case the manufacturer makes improvements. Either way I am much happier with my new A17 compared to the old one.