I thought this was fairly well established HF knowledge. One might even like the HD800s with a 100 ohms or more. Beyer tends to make amps with 120ohm output impedance for their products, it's no surprise that Senn chose a similar approach to power their 300+ ohm 800s. The HDVD800 is a shiny HD800 accessory that hobbyists have been demanding since '09. That demand might have cooled a bit since then but I don't doubt that the amp has a bright future.
Purrin's measurements with 170 ohm & 680 ohm resistors vs. near 0 ohm Cmoy: