Lior,
Despite what my signature say (my crack isn't done yet and I now use a mac mini) the HRT MS II (I have the Plus version) and the HD-600s are a very good match, and as John just told you, just need a usb cable and a pair of interconnects and you're ready to go -- the crack has a volume control, so no need for a preamp with this setup.
The upgrade to the HD-650 cable is a good one too. One of the things that will make a very significant difference is the software -- almost anything is better than iTunes by itself -- PureMusic, Amarra, Decibbel, and audirvana should all be considered (at different price and performance points) but all are a substantial improvement over iTunes alone. Also consider storing your music files on an external firewire drive, such as the Oyen Mini-Pro 800 for another nice improvement.
Then if you really want to take things up another several notches, consider the exter nal usb power supplies that some companies are offering -- this works very well on the HRT dacs as it supplies the dac with even cleaner power than the computer can.
I know some are skeptical about this and it doesn't work on every dac/music server combination, but with the HRT it works very very well.
HTH,
Jim
Jim Rebman -- recovering audiophile
Equitech balanced power; uRendu, USB processor -> Musette DAC -> 5670 tube buffer -> Finale Audio F138 FFX -> Cain and Cain Abbys near-field).
s.e.x. 2.1 under construction. Want list: Stereomour II
All ICs homemade (speaker and power next)