I have this and been using this for some time. Its absolutely an incredible amp and a beast in size and heft. Its very much comparable to 789, which was what I was using before.
Let me start by saying, there is something special about this amp. It's background is as dark as it gets, would be similar to 789. However, I like the sound of this amp more than 789. Everything just sounds sweeter, and more enjoyable. I started to understand difference between sweet sounding v/s clinical.
I strongly feel, this is a step up from 789 from musicality, and well justified at $400.
As with any high end amp, you need really high end DAC. Currently I am using it with D50, but planning to get D70 or M500 to really get most out of this and 789.