Just took delivery of the Bottlehead Crack OTL Amp kit and Speedball upgrade. Just ordered a pair of Sennheiser HD680 with a Moon Audio Blue Dragon V3 cable to go with the amp.
Can't wait to give them a try. I had a pair of Koss ESP-650 electrostatics for years, but didn't listen to them much for a variety of reasons (primarily long-term comfort and limited bass extension). Sold those recently.
Really looking forward to hearing what a top-notch set of cans and amp sound like, though it will be a little while before I get the Crack built.