I wanted to revisit this phone and what I'm using it with.
So my LG V10 is not rooted. I am using 90% of the 200gb SD Card and internal storage. Initially I did notice some increased lag time with some apps in their interface loading songs and track information, but that seems to have cleared itself up.
I started with Poweramp, which I used on my last phone. Obviously I have the stock music playing and the Google Music player. I've downloaded and tried a number of other music apps,and have settled on this rotation, that I like for different purposes: Stock player, DoubleTwist (paid), Rocket player (paid), Poweramp (paid), and then Tidal (hi res subscription), BubbleUpnP (paid), Youtube Player (trial).
I primarily listen to music in my vehicle, then at home through my stereo. I listen to music with my headphones about 10% of the time, although that's increased with this phone as I've been demoing more. I have both the Oppo PM-3s and Sennheiser HD600 cans.
Most of my music are FLAC files; I have a few albums that are Hi Rez FLAC. Mostly rock and guitar-based music - from the Beatles and Stones to Led Zep, Black Sabbath, et al, through the punk and metal movements of the late 70s and 80's (and on), 90's alternative, indie rock, folk, some hip hop, etc. My reference albums are typical Led Zeppelin I (Hi Rez), Houses of the Holy (Hi Rez), Tool - Aenema and Lateralus, Pink Floyd - Dark Side, and Converge - Jane Doe.
The Stock Player and DoubleTwist will use the HiFi DAC with no issues, no toggling, with my headphones, AUX cable, and through Chromecast (it's more different to analyze and compare with Chromecast, though... frankly I haven't spent a lot of time with this yet). So will Tidal and BubbleUpnP. Poweramp and Rocket Player do not, and beyond the volume differences, I find a significant sound difference between the apps that use the HiFi DAC and those that do not.
As such, I usually never use Poweramp (although it has the more nuanced EQ adjustability and definitely has a more processed sound signature than the other apps, even without the EQ or Tone adjustments on). Rocket Player is more neutral and natural - less overwhelming presence and bass, especially in my truck, with or without EQ adjustments. Rocket Player's EQ presets and custom settings are pretty underwhelming, though. It's okay to just set and forget, whether off or on, but stinks for adjusting. The more I fiddle with the stock player, DoubleTwist, and Tidal, the less I use Rocket Player, even in my truck. I mainly use Rocket Player because I like the interface the best, and I can Chromecast with it very easily. The other apps do not cast - Poweramp does not at all, DoubleTwist will only AirCast through Apple devices, and while BubbleUpnP does Chromecast, I find it a bit cumbersome with my loaded music, and primarily use it only with Tidal, which is the only way to Cast Tidal. It works well for that, however.
On to the important stuff...
First, there's a very obvious and distinct difference in the volume and sound quality when using the HiFi DAC and not, using any output option (stereo, cans, truck speakers). The volume difference is especially apparent with the HD600s, because they are higher impedance cans, they top out in volume pretty quick. The HD600s simply do not get very loud with the apps that do not use the HiFi DAC, and the soundstage is definitely less nuanced, more compressed, less detailed. With the stock player and DoubleTwist, the HD600s get loud enough for my tastes, but they won't get so loud that it hurts to listen to. In other words, the HD600s only get so loud, which may or may not be loud enough for everyone, especially those who want to push it to 11.
While there is an obvious and significant volume difference with apps using the HiFi DAC and those that do not when I am listening though the PM-3s, both groups of apps get plenty loud. With Poweramp and Rocket, you can get as loud as you want even with though you're not using the HiFi DAC; even louder if you are using the DAC with stock player, Tidal, and Double Twist. But there is still a qualitative difference in the sound, but with the PM-3s, because volume is not an issue, you can instead focus on what kind of sound you like - the customization you can get with Poweramp and its signature sound, or the more natural and nuanced sound you get using the DAC with stock and Double Twist. I still prefer those apps using the DAC.
(It should be said here that both the stock player and Double Twist have terrible EQs - and the stock player disables the EQ using Hi Rez files)
Comparing between the HD600s and the PM-3s... well, to me the HD600s sound much better. Again, more detail, separation, distinction with the open backed cans than the closed back planars. Not that the PM3s don't sound good - they do, and they get much more volume and punch than the HD600s, and maybe a little more attack... but the HD600s are just easier to listen to, less fatiguing, and it's really awesome to notice some of those details you don't quite get on a stereo or PM3s, little ticks of the symbols, the plucking sound of bass strings, longer acoustic guitar string ringing, etc. Somewhat subtle, but pronounced in comparison with the HD600s.
Lastly, I don't notice a great deal of difference between the stock player and Double Twist - both use the HiFi DAC, and I don't hear any differences in the sound between the two players. So it is more a matter of interface for me, frankly. And I prefer Double Twist. So ultimately I am primarily using Double Twist and Tidal to listen to music.
If I were to assign a score for these combinations, and keep in mind these scores are merely in comparison to each other, they are not objective scores, it might go as follows:
If HD600s using stock/Double Twist/Tidal (HiFi) is a 10...
PM-3 using stock/Double Twist/Tidal (HiFi) would be a 9...
HD600s using Rocket Player: 6.5
PM-3s using Rocket Player: 7.5
HD600s using Poweramp: 6.0 - 7.0 depending on the music
PM-3s using Poweramp: 6.5 - 8.0 depending on the music.
I hope that helps. Maybe sometime I can test using some very cheap earbuds just to see the comparative qualities therein.
In sum, the HD600s are really, really good with the HiFi DAC. Even with the volume limitations, I find myself using them so much more than the louder and punchier PM-3s, unless for some reason I want to listen to music with the apps that aren't using the DAC.
But more importantly, I feel like the HiFi DAC is a much bigger and more important influence on the sound quality than the headphones, as long as the cans you are using are adequate. It is noticeable and significant.
Just to add a little more context to my review, I feel like the LG V10 keeps up and maybe outperforms my iBasso DX90. I have since returned the iBasso. However, the LG V10 starts to lose ground when I compare with my home stereo system, especially with the HD600s, which are just much easier driven with the preamp. And I'm not using super expensive high end gear - my home system is a Sansui G4500 receiver, using a Schiit Modi 2 DAC, playing FLAC files through J River Media Center, or through my Denon D46 turntable. But I find it's hard to really compare a portable source player with a dedicated stereo system.