I bought my unit for 300Euro (used). But they were quite expensive when they were tested in the ausio magazines some years ago.
There are versions for stax (202/303/404 Lambda Signature...SR007...)
Sennheiser HD800 as far as I heard.
AKG k1000 and some more...
PS: I posted some pictures of that unit on the stax thread.
I know, this thread is a little bit old but where is the mythical DF equalizer for the Sennheiser HD800 you speak of?