Shit, that sucks. You must have the worst luck ever.
I'm pretty happy that my new pair doesn't squeal. I don't have anything other than the Koss amp right now though, so I can't validate faust2d observations, but from what I remember, when I had the KGSS and SRD-7Mk2, the squeal was present no matter which amp was used.
The ESP950 are surely not plug and play headphones... It's almost necessary to recable them and buy another amp and apparently they are not built well. In the end, I enjoy these cans quite a bit so I'm keeping them, but they're not "user-friendly" if such a thing can be said about headphones.