MrGrimbleLg v20 already has a very good sabre DAC , get the topping nx3s which sells for like 60 dollars and has a maximum output power of 540mW almost twice as that of the nx4 . That way you can even drive some of the most power hungry headphones.
MrGrimbleI don't have particularly sensitive ears, but I tried using the NX4 with my LG V20 and Sennheiser HD6xx headphones - I cannot hear a difference.