> It is slightly boring combination

This is something I've experienced with a lot of amps and HD6x0's. Tube amps are often recommended for the 600's. I personally thought the Jot2 was an interesting pairing using balanced. 789 was okay too. Also paired nicely with r2r-11mk2 Something about the r2r-11mk2 and HD6x0's just work very well for me.