I owned the X260.8's (and XP-22) for a few years and loved the sound they provided - clear, warm, and impactful even at lower listening levels. I never owned the .5's, but did hear that the older version was a little warmer, compared to the .8 counterparts.
I also heard that folks preferred the .5 amps for the XA line (class A), but the .8 amps for the X line (class AB). Small tidbit, as of a couple of years ago, Pass would still make the .5 amps for customers. I would imagine that is still the case but don't know for sure. Hope this helps.