@stuartk ... well, you have to be happy with the amount of responses; that's encouraging, on some level.
I have a dac that's close to 14 years old. It developed a timing error that resulted in overly-clinical, often sibilant presentation. Apparently, it was over-reading ... spending too much time reading parts of the digital chain. So high frequencies were over represented. While it still sounded great I'd often get headaches from Miles Davis' muted trumpet and the like. I called the manufacturer, which helped diagnose the problem ... I'd definitely get the Aqua folks involved in this caper. Those guys are brilliant! Good luck, my friend ... Happy New Year!!