I have this DAC/AMP, only thing I don't like about it is the rated claimed output power does not match with what I anticipated. 910 mW / 32 ohm sounds to me more like 300 mW / 32. Tested on Fostex T50rp MK3 and other headphones. Great features though for the price.
OxygenIt is just enough for MK3's. But it's good enough. But arrghhh could of used a bit more amp volume.
But as a example regarding output power.. my FiiO E12 Mont Blanc Portable Headphone can output more volume in high gain (with bass boost off) than this amp. (Tested on my PC - E12 was on headphone out and X6 on optical and usb) The return would of been too long since I had purchased it on AE, but if I had the choice I would have gone for the Monoprice 111567 dac/amp that has about the same "advertised" power as the X6 and I imagine that one to be truer to the power specs. I also used my other headphones such as SHiPs 9500, Senn HD598.
So it's a great amp for the price. Though can be had cheaper and with free shipping elsewhere but you would have to wait at least 3 weeks for delivery from China and the returns would be more of a hassle.
Other choices that I would try would be the Schiit Magni 2 Amp and Modi Dac.
Other amps I own and happy with are the FiiO e10k, and the FiiO A3.
I have a mini overview of the X6 on head fi. As I said, the main disappointment for me is that the power specs are way off IMHO.
I use my headphones and amps for Music, Gaming and Video sound editing.