Ok, distortion pattern looks like tube, is this good or bad? ;-) Maybe this is in the nature of the Burr Brown DAC chip when running it usb powered, I have no idea. All I know is that iFi themselves emphasize the official specs is while running it with a power supply.
Well, I try my best to relax and enjoy my Zen Dac (running 5.3c firmware and a power supply) together with HD6XX. If reading reviews and looking at products every single day this hobby would probably drive me totally crazy. ;-)
I have been enjoying listening to music all my life but this is a new interest for me (better sound through headphones), if buying today I would most probably get the SMSL M500 instead. The Darkvoice tube amp for $200 from Drop is really good value too I think (?) but it's with no DAC included.
Anyway, for me the Zen Dac together with HD6XX is sounding really good, I can't complain. You can possibly get a tiny headroom issue if you EQ it a lot or listening a lot to classical music but I think it's ok for me, 300 ohm headphones definately the max for the Zen Dac though (and this is while running it with power supply. In addition I think the exclusive mode in Tidal is giving me little bit of extra hedroom comparing to Spotify which is interesting.
Perhaps both Zen Dac & HD6XX being slightly smooth, warm and a tiny rounded treble peak in their sound giving me a double effect of smoothness. I assume it's the Burr Brown DAC chip (DSD1793?) being a bit warm in it's nature? It's the opposite of ear fatigue though which I really like. Probably helps I don't have much to compare with and I'm not planning on visiting any Hifi stores in the nearest future. ;-)
Usually playing Tidal Hifi & MQA tracks but I have been trying some flac files stored on my computer too enjoying Pink Floyd, David Bowie and classical tracks. I have no idea if MQA is good or bad but it sounds really good in my ears when being rendered through the Zen Dac.
It's a little bit annoying it's not possible EQing the sound when running exclusive mode in Tidal unless buying Roon. I was playing around with oratory1990 settings using foobar2000 while running locally stored flac files and I liked it.
A found a good summarize in this review (not done by me):
"Thoughts / Conclusion:
Well, I have to admit, the Zen was on its way to being the 1st iFi product that I couldn’t honestly recommend. It has limited output power, limited input options, has a fair amount of noise and even a bit of hiss on high gain, and lacked a bit of detail to the signature that I would have preferred. Based on that, I was going to have to say pass on this one until they do a bit more work, but then two things happened. First they did a bit more work and the new firmware with the GTO replacement for the linear filter is a big improvement. So much for my too smoothed over argument. Then I plugged in a linear power supply and my other major argument about noise went away too. I was left with a good little dac / pre-amp once those two issues are cured."