The Pixels and the HTC 10 use the exact same DAC, however the voltage coming from the 10 is approximately double that of the Pixel and thus it has the ability to become louder for wired headphones that draw more power than the Pixel can push out.
The vast majority of devices are between .26v and .75v, while several recent devices are between .96 and 1.00v - which is where the HTC 10 falls. Based on where over 220 of the data points fall, my best guess is that the average pair of headphones is using somewhere very close to .4v to .5v and that higher quality headphones are going to be pushing up towards the .8v to .9v, however I haven't been able to find a good source to get that data from.
What this would mean, if accurate, is that almost all devices will get very similar if not exactly the same volume with run of the mill headphones, while a select few (most of the devices up in this area and above are tablets) will get higher than average volume from higher quality headphones.