UnstableFluxThe HD6XX are still made in Ireland. They moved the K7XX to China and the build quality shows it. Supposedly the sound is still decent, but I don't like the little elastic string for headband retention... seems cheap.
UnstableFluxThese are more neutral sounding and has a wider and airier soundstage. The K7XX requires less power but is more amp picky, you really need a good quality DAC/amp for them like something from Aune.
UnstableFluxThe K7XX requires more current to drive, has more treble (sounds more airy/analytical), has a larger soundstage, There's a dip around 3-5kHz that makes cymbals/wooden instruments sound slightly more hollow. The HD6XX sounds more "intimate" and "easy to listen to" due to the smaller soundstage and less treble. The differences however are pretty minor and I'm not really sure which I like better.