I have used both the Headroom Supreme AND the Creek with the HD600's, and the BOTH choke.
The HD600's/580's are very difficult to drive. They are high impedence, and have a big impedence jump about 100hz, which knocks out their low bass with most amps.
I have settled on the ASL MG Head OTL as my headphone amp.
Makes the HD600's tolerable. The transformer out give more detail but less bass. The OTL output is ok.
I plan on trying some Clou cables soon.
Oh yeah, if the MG Head had not done the trick, I would have dumped the HD600's.
Lack of low end bass (less than full range frequency response) is unacceptable in a headphone of the HD600's claims and price.
You have to use a high impedence output amp.
Good test for low end response? Common song-"Lady Marmalade" by Cristina Aguilera. It has some really low notes, most systems won't reproduce. Also, Reference Recordings label "Mephisto and Company" (classical, Eije Oue and the Minnesota Orchestra), the track labelled "Baba Yaga".
I use the Baba Yaga to test speakers. One speaker salesman said "I've never heard them do that before" when testing Vandersteen IIce Signatures.
Once I got speakers that had TRUE low end response, and also had the Sony MDR7506's, I got used to low end sound being present. Then I went on a headphone quest..dumped the Grado's and went into "fix HD600" mode. Have new headphone amp now, and am going to try the Clous, as soon as Headroom credits my card for some returns.