Originally Posted by Manichean
Why not use ePub? LRF is no longer supported by Sony, and the Calibre plugin is, accordingly, no longer maintained. The devices you mentioned should support ePub as their intended "native" format.
EPUB in both PRS-300 and PRS-600 doesn't show justified text, which is a feature I like. Both models date back to the times when Sony supported primarily LRF, and actually perform better at this format (though PRS-600 is pretty good at RTF too).