Their reviews suggest even the latest V30 with Quad DAC isn't better than mobile phones such as the LG G5 and Huawei Mate 8 on average and on a variety of factors
That's the G5 with the Bang & Olufsen "Friend" attachment. Basically a USB DAC-HPamp with low power (ie, not that much more power than a mobile phone) but clean output. Look for the full review on the G5 and measurements are horrible without the B&O accessory. Makes you think it isn't so much B&O (considering the performance of the V10 and V20) being good at it but more of LG trying to convince people to shell out more money.
Mate8 isn't bad either and there are China only phones that can't get US regulators' clearance for some reason that basically a dedicated DAC chip, output opamps, and headphone driver op-amps like a FIio X3, just without a dedicated line out from the output stage op-amps, instead of the combo DAC-headphone driver chip you see on mass market devices.
Also the "Quad DAC" only activates to drive a headphone with all the chips involved (ie think of it like whether your CPU intensive app can even run on multiple cores and threads, unlike how older Total War games go faster on an i7 because of single core computer performance, but will get slower on the octacore i7 vs the higher clock quad core) if it detects high impedance, which is kind of idiotic since sensitivity is just as important and there's no way the phone can detect that. They likely did this so they won't lose battery life tests.
I get a DAC can help amplifier higher quality headphones with minimal detriment to audio quality...
No, a DAC doesn't help amplify headphones. A DAC is just a Digital to Analogue Converter. Its job is to make 1110010000010011100000111000101011 into an electric signal that the amplifier amplifies as it moves the diaphragm which then moves air to make sound that you hear.
On mobile phones and tablets what you get is an integrated audio chip, basically a DAC chip has the headphone driver chip built in, so if it was 1990 that would be like if Philips put the TDA1541 DAC and TDA1308 into the same die the same way AMD thought, "but what if we get an Athlon II CPU and an HD6600 in the same die?" The difference is that the TDA1308 is a high power chip for home amplifiers, while the phone needs to save battery life, so the latter outputs somewhere around 5mW to 15mW at 32ohms. Still similar limitations like no high current power supply with large caps on the audio chip while the APU has to share the RAM between the CPU and GPU cores.
Basically "Quad DAC" is a term dreamed up more by the marketing department because "multi-drive amp stage on audio chips" can't fit into a logo. Basically, it's like the process for simplifying something down to a meme. Just look at the box on electronics devices.
...but what about mid-range headphones. Would you get better audio quality from the G5/Mate 8 than the V30 and such?
Obviously, it would be better to get high quality headphones with a phone that can power it (V30) and of course, mid-range headphones that don't require lots of power would be bottlenecked by it's audio quality.
That depends primarily on the sensitivity and then impedance than the price bracket they're on. Problem with the Quad DAC is that it activates at a certain impedance rather than take driver sensitivity into account, so while a low impedance headphone theoretically would be at the meat of the powerband of any amp circuit that is not an output transformerless tube amplifier, you'll either not be using all the power of the Quad DAC or you're using it on a headphone that really needs more voltage than what you can get out of a device running a CPU and screen on top of the audio chip off a 7v battery. So basically it will drive an HD600 better than a Note8 can, but it's still not as good as with a good desktop amplifier. And since an open back high impedance headphone isn't actually realistic for walking around in, chances are you'll never really use the Quad DAC feature.
In other words - I'd still much rather use a 100dB/1mW or higher sensitivity IEM or, at worst, a closed back headphone, even if it's not for the LG Vx0 phones.
But let's say, I only have headphones that don't require an amp to reach it's audio threshold... WOULD the G5/Mate 8 really be better at audio? Perhaps newer phones have too much tech packed in that interferes with the audio hardware?
Maybe slightly better imaging given the lower crosstalk.
Personally just choose a phone based on your other needs and what you can afford, just staying the hell away from phones that have horrible measurements on the 3.5mm output.
