I'd also like to tell people who own the newer versions of the Apogee Duet (mine is the latest Mac/IOS version), that the HD600 works beautifully with it. Of all headphones I've used with it, the HD600 is the best by far.
If it hadn't been for some people saying otherwise and making me over-cautious about amp requirements, I'd have bought the HD600 months ago and wouldn't have wasted my time buying and auditioning many alternatives, like the K550, DT880, HD598 and others which proved nowhere near the class of the HD600.
I don't doubt that other amps can be even better, just as there is always a better amp to drive speakers even if one has already spent $5,000 on one. But to my ears the Apogee/HD600 is a fine pairing, smooth and controlled, wonderfully musical, with plenty of power. Fom my recollection, it is a MUCH better combination than the HD650/Musical Fidelity X-Can v3 pairing I had some years ago. So my recommendation if you have the Apogee is to make the HD600 a prime consideration, whether or not you intend to upgrade later.
The Apogee also drives my Neumann KH120A studio monitors and there is a very nice complementary sound quality when switching between the speakers and the HD600. VERY happy with this setup!
Edited by wrightanswer@ear - 10/8/13 at 11:57am