Midgetguy
Headphoneus Supremus
I totally agree, the HD600's are so nice I'm having a difficult time trying to find another nice headphone to compliment them.
Fidelio X1 adds more bass and basically gives your head a hug lol. So depending on your music, it has some complementary qualities. The HD600s just do a very good job at pretty much everything though, it's hard to find something to complement.