I have a Headsets Inc (HI) unit installed in a David Clark H10-40 that
also seems very acceptable, although I've never compared it to any
other unit. As I understand the HI kit operates on a completely
different principle, being an analog servo-cancelling vs the digital
iteration schemes from others. I'd think there would be a major
difference in performance somewhere.

Has anyone else compared the HI modification to one of the all-digital
ones?