Daffa,
I've owned a Blockhead for awhile now, and I can assure you that I hear no viel with the Senn 600.
To the best of my knowledge, you have to buy the RS-1 from Headroom already wired for balanced operation. Although I suppose you could wire it up yourself, with enough soldering skill.
Again, to the best of my knowledge, there are only three officially approved headphones, the Senn 580, 600, and the Grado RS-1. I've tried several times to get Headroom to wire up other phones for me, but they don't seem to be interested.
One thing I did was buy a cheater plug from Stephen Audioart. With this plug I can use any headphone with the Blockhead. I'll tell you that the Grado HP-1 sounds quite nice run this way. I don't think fellow head-fier, and Melos expert Carlo likes the Blockhead all that much, but he has listened that way, and can probably share some thoughts if he'd like.