The Reußenzehn is more like a finessed Italian engine, while the King is all USA musclecar!
Interesting design, but all of that clear acrylic completely kills the aesthetic for me.
I've seen them at an audio show, and frankly they don't look good at all...
I disagree, as the HP8 that I listened to recently sounded very good indeed.
John Grandberg (project86) reviewed the amp very favorably too - http://www.innerfidelity.com/content/icon-audio-hp8-mk2-tube-headphone-amplifier - in this interesting and informative article.