<VERY long time, no post...>
I just got mine, and spent some time digging around before I figured out I had to activate the DAC in the settings before it would come on: Settings > Sound > Sound Quality and Effects, and toggle the High Quality DAC switch. Once done, the phone auto detects the headphones. Maybe you need to do that, too.
Reference:
https://www.phonearena.com/news/How-turn-on-and-make-sense-of-the-LG-V30s-Quad-DAC_id97750
With 250 Ohm Beyerdynamic DT990's It sounds better than the Dragonfly/Jitterbug combination I have on my Laptop. Even with the exact same source material (I copied my ripped files over to the phone). I also tried my Oppo PM3s (20~something ohms???) and KEF M500's (32 Ohm). The Beyers easily sound best here, followed by the Oppos, which sound very good. The KEFs sound allright, and I can tell the difference when toggling the DAC on and off in the system tray. The phone definitely likes the Beyers. The one thing I noticed, however, is if you want to crank up the volume towards the upper reaches it starts to sound busy and somewhat compressed. So, best at middle volumes. Still: Light Years ahead of the Samsung I traded in.