Can you check if the Epub render of the 600 has the same 300k per flow limitation of the previous models? I just posted a book for you to try out if you can
I downloaded a mobi ebook from
feedbooks and just converted it with
calibre. I increased the 260k split in calibre to 1024k. Its just a quick and dirty convertion so dont blame either feedbooks or calibre on any errors or ugly looking formatting.