i have the same issue on my rampage black board, there is definitely an issue going on with some of the drivers due to customization of the board hardware(for no reason, the NEC relay clicks and pops on startup and transient loads, the headphone amp sucks, the wima caps were only ever used on this board and no other board by either asus or another company for good reason, the fact that this implementation of the "Supreme FX 2014" was never used on any other board combined with asus happily abandoning us means we are seriously out of luck, cheap entry boards with stock ALC1150 audio solutions punch this boards audio in the face.
i had an issue with the onboard sound poping and crackling
https://rog.asus.com/forum/showthread.php?71087-Why-has-ASUS-Forsaken-the-RIVE-BE&p=518707&viewfull=... this issue went away when i upgraded to windows 10
i also tried the generic realtek drivers and got unbalanced audio like you are describing, and still suffer it with the latest drivers, i thought i was alone on this issue, i have not been able to solve this perhaps some are more picky than others as it is only slight, however this audio solution on this board is a disaster and by far the worst i have used in some years, i am at the point of purchasing a DAC , as asus will no longer offer these customized drivers, the drivers they list on their site now are generic realtek drivers that will install and then display an ROG theme, but they are not a solution. the audio on this board has always been crap it is very muddy and has no midrange warm, with an excess of bass and volume leading to poor control, playing audio on this board through my Grado Reference Series RS2e headphones, i can notice the volume difference in each channel, and it sounds extremely harsh compared to the same audio through an htc one m8 & iphone 6 or even my other boards with any realtek chip from 2010 onwards, using the front case headphone jack which uses the onboard headphone amp is ridiculously loud and harsh, i have given up on the audio on this board, which was mostly the reason for purchasing it above other x79 boards
😕