That's a hard question to answer. So I will give you more than one choice.
Now, according to ppl, the AD843 and the EL2009 combine to make the best opamp/buffer combination for a headphone amp or preamp. The EL2009 and EL2008 are discontinued and hard to find. However, Tangent sells the EL2002 at his Audilogica site.
Definitely stay AWAY from Burr-Brown op amps when using the Senn 600s!!! The AD stuff tends to help with the midrange a little bit more and have a better sound IMHO!!! You want to pair up some good AD stuff like the 8610 for single channel, or 8620 for dual channel, or go with the AD843 or 825s. LC Audio sells some modules that come with the 825 and some decuopling caps that really sound nice in my set-up.
I'll let someone else speak about the buffers as don't have any experience with them myself.