Does not fit the variable slope HP requirement but this looks interesting
http://www.audioc.com/accessories1/rdes/rdes.htm
A bit more discusson here
http://www.audiocircle.com/circles/viewtopic.php?t=13650&highlight=
http://www.audioc.com/accessories1/rdes/rdes.htm
A bit more discusson here
http://www.audiocircle.com/circles/viewtopic.php?t=13650&highlight=