While I didn't get a chance to compare them side by side, after auditioning both at different times, I felt the XD has a bit more treble and is more airy, open, and extended. In other words, it's gradually creeping towards a more "modern" sound profile. Some might like it, but some undoubtedly will prefer the classic Harbeth sound. Having said that, the differences are subtle at best. IMO, of course.
