The new Sennheiser flagship is a closed back design. Just how good is it ?

When Sennheiser release a new high end headphone, everyone in the audio world takes notice. It was therefore with great interest when they announced and subsequently released the HD820 that we have here for review today. Not because of the price jump to £2000 (from £1400 for the HD800s), but actually more that it was a closed back design. I certainly wasn’t expecting that.