It's unanimous how good the Mjolnir is for the Audez'e line but there are very few here who use it to drive the HD800. The assumption is that it is too lively for the Senn. That is what I found too when I had it paired with the Gungnir. Upgrading to M51 revealed the Schiit dac as the culprit, with the Mjolnir simply presenting the source as is - no gloss, warts and all. With the M51/MJ pairing I get soundstage width, deep but tight low end, explosive dynamics, and zesty upper mids and treble - without the obnoxiously loud and compressed dynamics, and fatigue inducing grain I suffered from the Gungnir/MJ combo.
The Mjolnir is a wicked amp for the HD650. It turns a quaint indie film into a balls to the wall action blockbuster. I'm not sure how the HD650 could sound better.