Two years ago, Sennheiser sold, through Meier Audio, around twenty brand new HE90 units.
After several buyers mentioned they had problems (because they used the HE90 on other amps than the Sennheiser HEV90 amp), Sennheiser agreed to modify a bit the headphones so that they can work on any electrostatic amp on the market (because, if I understood correctly, the HEV90 amp had a very different bias than most of other current electrostatic amps).
Does anybody know exactly what were the modifications made by Sennheiser at this new batch of Orpheus headphones?