Although it is not available in the US yet, I picked up a Sony PHA-2 amplifier.  When using the line out on the D6C, the sound is incredible.  Although the D6C sounds great on its own, Sony is working its magic with the PHA-2 and somehow manages to improve it even more.  This is the first portable headphone amplifier that I've used, and if you haven't used one, I'd highly recommend it.

It's a little weird mixing old technology with new, but it sounds great!

I red about the Sony PHA-2 a while back, the only problem here is the PHA-2 has a built in DAC a bit redundant with D6C. There are many options out there with just analogue section.

I use the Ray Samuels P-51 Mustang 

Looks good that SONY...

Anyone heard about this super-cool Carot One headphone amplifier?

http://www.carotone.it/ernestolo.php