Regarding headphones I tried it with AKG K92 and ISK HP 580, and it sounded really great. Lots of power and enough clarity left nothing to be desired. Just to put this into perspective, SMSL Idea/Sabaj Da2 or Sabaj Da3 couldn't provide same amount of liveliness and fullness to the K92. X6 actually sounded closer to more expensive Topping NX4.
With my home setup, it just happened that I was transitioning from one amp to another, from older Harman Kardon HK6500 to a newer and much more transparent Cyrus 8vs2. On my old amp I could notice Topping D30 and NX4 sound better but difference was just not that important to ponder over it. Later on when I switched to the Cyrus I realized that D30 and especially NX4 gives cleaner and livelier sound. I can't put numbers on these subjectively observed differences but let's say that FX DAC-X6 did not embarrass itself in any way, especially when you count in the price.
So in my book and for this price this is a solid 5 out of 5 product.
As usual lately, I made a video review too.