Your both headphone have special coating on their jack, that's why it triger the quad dac.
I am very sorry for misleading you. Those Sennheiser HD429 and Harman Kardon Soho are recognized as Normal devices so only one out four DACs is activated. Since both headset are decent for their 32 ohm sound quality is good.
But. I've just tried with Beyerdynamic DT 990 both Pro and Premium 250 ohm and discovered sound without any background hiss. Absolutely pure crystal sound. And yes, headphones were recognized as Hi-Fi.
sagor1 is totally right. If you want to get the most from QuadDAC equipped handset - go for 250+ ohm quality headset.
BTW I've got Samsung by AKG earbuds. I am sure these earbuds are best suited for LG G6. I compared to stock LG earbuds, Sennheisers and Beyerdynamic ones - Sammy's AKG is the best. So if you find it under 30 bucks - buy without hesitation. I don't actually think that investing in any earbuds heavily is wise especially if you plan to use em with fantastic QuadDAC handsets.