Weird, I can't add this to the review section, or find a way to give star ratings, so I'll put this in discussion...
This is a review of the X7s with the HD6xx's. I'm lucky, I have a big boy hi-fi. It is a Linn MDSM/2, and I also have an Oppo 105D. In other words I have a couple of world-class DAC units. I have single strands of cable that cost more than this amp. I know that sounds dickish, but all I'm really trying to say that hey, I have a good system, and good ears.
I bought the X7s because I wondered if I could improve on what I was getting out of the HDxx's from the 6.5 mm jacks of my two incredible DACs. To make a long story short, when I connect the amp via RCA cables (Linn blacks) to either one of the two DACs, it does make the headphones sound better. More grunt, more slam in the low end, a little bit smoother, just generally more of everything, in a good way. These observations come from using a stock Senn 6.5 mm headphone jack on all three units, the two DACs alone, and then the amp connected to each of the DACs and to my LG v30 via a 3.5mm to RCA adapter, so five different conditions. Presently, my favorite condition is the Oppo into the Aune. It surprises me, the Linn does deliver a little bit more detail, but the Oppo just does a better job at putting out a little bit more sparkle, which I think the 650s need because they're so ....650. Maybe that'll change later as I swap back over to Linn, but I'm really enjoying the Oppo to Aune right now.
Linn's concept of how to build a hi-fi is, "source first". So for those of you thinking about whether to buy the combo or the amp alone, if you don't have a good source, think about getting the combo first. It seems many commenters here are saying the amp in the X7s is better. But consensus holds the DAC in the combo is pretty good. Use the combo as a stepping stone, sell it when you get a little bit more money and buy a stand alone DAC, and the X7s. And then keep going in a never-ending upgrade cycle.
I'm now almost exclusively using the XLR output on the Aune amp. I got a $45 XLR cable off eBay, it's decent enough with the overall setup value, and I will comment that the XLR cable has increased the output, and the general goodness, of the system.
I'm burying the lead here, because my main purpose for writing this review was to speak to the issues several people here are commenting on.
I also am having hum/buzz issues with the amp. I bought the above XLR cables for the 650s, and now when I use them this hum is even louder. I get no hum when listening directly via 6.5 mm from either of the excellent DACs. The hum is also much less when listening to the 6.5 mm jack on the Aune. Because of this I'm guessing Aune has some issues either with its board or its power supply.
I did an interesting test where I went around my house, turned off everything in the panel except for my hi-fi circuit (and all the hifi equipment), and then listened to see if I could decrease the hum. No improvement. I then turned off everything at the breaker except one of my (ungrounded) kitchen splits, plugged in the amp and still had the same amount of hum using both jacks on the amp. Clearly I have some garbage power in my apartment building, which I guess is par for the course given the circuitry in an apartment building. But then I walked 2 m outside my apartment door, and plugged the amp into a dedicated circuit for the fire department. This immaculately grounded, never used circuit, had completely no hum, and was inky black on both the Aune amps 6.5 mm and XLR outputs into the 650s.
Another poster here has linked to an improved power supply that Aune is now offering. Given that the rest of my hyper expensive system doesn't exhibit this problem, I'm gonna guess that Aune has made a nice $200 amp, but has some serious compromises in its power supply or other parts of its topology that make this unit susceptible to dirty power. Maybe that's why this different power supply is now on offer on their website. So if you got hum issues with the amp, you're not alone, you're not crazy, it's clearly a shortcoming in this device.
I've been kicking around the idea of getting a proper power conditioner for my big boy system, and I've decided that I'm going to take a little plunge and start with something called the PS Audio Dectet. On the rare occasions I use the headphone set up I would like to have this hum issue eliminated, and it probably wouldn't hurt the rest of my system. If nothing else it would provide a decent amount of protection from this noise, and more importantly power surge issues. I'm also going to drop some money on some nice Hubble plugs for my whole hi-fi circuit (sadly not a dedicated line), which people say is also helpful in the never ending search for perfection. I'll try to report back here on if it helps the amp hum in any way.
So if you've read this far, yes, buy the amp. I find it amazing that, to my ears, it can improve on the sound from my two spectacular DACs. Both units have dedicated headphone circuitry, and do a great job on their own, but the Aune X7s amp adds just that little bit more. Is it 200 bucks more? Yes.
Edit: I now have the Dectet, and the hum wasn't reduced at all. I'm now finally going to organize a proper ground from panel to equipment, and all the other outlets on the circuit. Maybe this sensitivity of the Aune to crappy power can be viewed as a feature...a power analyser?