I don't know what could be causing this error you're getting, but I flash 512kb BIOSes onto Duo-X2's through EvoX all the time without a problem...they are definitely 512kb, I edited them in Evtool/Xbtool myself. This works fine for me with both the M8+ and X24981 BIOSes. What version of EvoX are you using? And what BIOS are you experiencing the corruption with?
RustAngel