Definitely much higher volume of out the computer. I guess the line level for the Nexus 6p is much lower. No idea how to increase it though.
This is a copy/paste of my reply in
this thread.
"Concerning volume issues with Android, I'll give a quick run down of my experiences so far.
Android system through USB OTG is limited to 15 volume steps regardless of app used. This applies to Google Play Music, YouTube, MX Player, etc. I find that the volume output is quite low, even with the 8 ohm Dunus, and I must use volume step 13 out of 15 to get listenable levels. Strangely, the volume difference between steps 14 and 15 is significant! It's like going up 4 or 5 volume steps at once. Perhaps max Android volume = Line Out?
USB Audio Player Pro offers the most amount of volume control although the software volume is limited to 20 steps. However, the amount of volume allotted to the 20 steps is adjustable to varying degrees when combined with the Hardware Volume and EQ gain sliders. I haven't calculated how many levels are available in Hardware Volume, but the slider moves in small increments and I have yet to go past 60% to attain optimal listening levels with my gear. I don't have any 300 ohm headphones to test, but with how loud it drives my Q701s I imagine it won't be a problem.
Poweramp alpha build 703 allows the user to adjust the amount of volume steps in software. The levels are 30, 50, 76 and 100. It works incredibly well but, on my phone at least, only works when Poweramp is open in the foreground. When the screen is off the volume steps will default to Android's 15. A neat feature with the alpha 703 build is variable output options including a hi-res option for phones that support it. I don't want to delve too deeply into the options here as they are a bit complicated, but sound quality on the alpha build is better than the official release and even its DAC output sounds better. Volume is still a problem though. Using the Poweramp EQ I have to drag the preamp up to full and put volume up to 75%+ for listenable levels with the 32 ohm V-MODA M-100. Sound is just as good as UAPP, but the volume control isn't there.
So, for the time being, UAPP is still the best option for Dragonfly."