I've owned or tested the LG V10, V30, V30s, V35, and V40 and i can tell you there was a definite sound difference at the V40 split (when they went to Meridian tuning). It is smoother and less bright/clinical than the previous V's--and its noticeable w comparison on quality headphones especially with material like early Beatles. The V35 is where they started adding (some would say unnecessary) audio options like DTS-X and EQs--but really help when playing in the car.
Even though i am not a huge fan of the V40 size increase, I use that as my DD over the V30s because of sound difference.
I'm personally wondering about the audio on the new G8X, about to be released in the US with that second screen included for only $700 unlocked. I hear it uses the same DAC as has been in place years now....but wonder if anything else is different?