Hey guys,
Turns out I'm a big dummy. The sound controller is a SoundMAX chip, not Realtek. I got the 6.10.2.6570 SoundMAX driver from Asus support and had to slightly change platform.cfg to get it to install. If anybody had a similar problem, I'll elaborate. But either way, hissing and squealing are gone! Finally have all my devices working 100% (2 months later)
Thanks anyway!