The ESP950 is seriously awesome, I'll probably end up picking up a set sometime. Frank, I remember somebody (I think cetoole) opening up the E90 and checking the bias voltage. I think there was some conflicting info that showed a different bias for different E90s. Do you think the squeal might be a result of using the wrong value bias?
I'm the one that bought cetool's pair. I think there is some discussion of the E90 bias value on other forums but I don't remember the conclusions. I think the normal Koss bias is 620V. I seem to remember something about lowering the bias voltage to eliminate the squeal, but I could be wrong. I run mine at standard Stax 580V and I still get an intermittent low level "squeal". Driver replacement cures the problem.