Here's a crop of the top-right corner of my cover, I've zoomed in to 200% and it's shocking how blurry the amazon image is. It looks like they use really high compression. Bummer.
So either I need to start a new design with fewer geometric shapes, or I need to find out if I can get around this compression issue. I've tried it as a TIFF rather than a JPG and it's somehow inverted the colours. Oh well, it looks ok on the Kindle Fire and iPad preview, maybe the inversion is fate.