I would opt for the 5 if you're using with D7000 headphones. I think the Master-6 is really aimed at HE-6 and LCD-2 owners only, and I do think you'd be stuck at a very low volume setting. I don't even need 9 o'clock on my NFB-10 to drive the HE-6, and it has very similar output power to the Master-6. The 5 should sound better just because of the class A power supply.
Edited by somestranger26 - 9/21/11 at 12:58pm