The HE90 driver is quite big but according to the pics from Senn, the diaphragm isn't that big. I did measure the drivers once but can't for the life of me find the measurements... The Jecklin drivers are bigger at roughly 100mm square.
His unit had all the usual issues, a single filament supply, outputs at 100v and iffy build quality but far from the worst I've seen. As for buying an ES-1, are you nuts!!!!??!!?? ... The T2 and KGBH would both be excellent matches and I preferred my old BH with the Omega over the 007 (which liked moar powahh!!!). Never got a chance to try the hybrid with it though...