[shamanNS]

@dzhmartin: Since you can't jailbreak your Kindle only option is removing all KFX books from your Kindle and then using your PC to re-downloading all of you books one by one (since the "Download & transfer via USB" option isn't available once you have selected multiple books) as azw3 using ""Download & transfer via USB" option on "Manage Your Content and Devices" section of Amazon's website.

That option forces a book to download in azw3 (KF8) format instead of KFX format.

Once your book(s) is in azw3 format then the FONT_RAMP thing is in effect.

Alternative approach to "download one by one" would be to use Kindle device or Kindle app that doesn't support KFX to speed up the "download all books" step but then you would have to remove DRM from each book to be able to read it on your Kindle Voyage.

Common step in both approaches is the last one: transfer azw3 files to your Kindle via USB cable.

And for any new books you want to send to your Kindle in the future you would have to repeat "Download & transfer via USB" > manually copy the file to your Kindle.