The gain and the number of channels are pretty much unrelated. You can build a 2 channel with 2x gain or a 3 channel with 2x gain. However if you get a 4 channel balanced amp built, then the gain will be doubled. I am sure there is a way to get a 4 channel amp built with 2x gain, but that would mean each board must have 1x gain, and I'm not sure how to do this (usually it would be leaving out the gain resistors, not sure if it would work in this case).
I went the simplest/cheapest way and built a 2 channel amp. The gain you want won't affect the price of the amp, but must be specified as there is no good way to implement a gain switch. The amount of channels you want will affect the price, by a good amount.
I'm in the same boat as OP, and right now I'm considering the Schiit Asgard 2. I already have a Cambridge DacMagic Plus, and I'm content with it as a DAC for the time being. The built-in amp however, while very good for the price, I think isn't doing my D7000s justice.
While cheaper than I had really considered, the Asgard 2 seemd like a good choice after reading Macick Man's post in a similar thread.
Well, frak. Here's the word from Schiit:
It won't damage the D7000s if used properly, but we wouldn't recommend it for another simple reason: Lyr is a very high-gain amp, intended for inefficient orthodynamics and high-impedance headphones. It'll probably have too much residual noise for the insanely sensitive D7000s. We use Asgard ourselves with our D5000s, and it is a perfect match.