Does dolby headphone sound better than stereo? Absolutely not. Does it provide better positional audio. Probably.
Any time you subject your audio to some sort of equalizer or digital processing it degrades from the original source. I use my soundcard in audio creation mode even when I'm gaming, and I refuse to use CMSS3d because it sounds horrible.
Well I think by quality in the competitive sense, positional accuracy trumps stereo reproduction and clarity anyday... I wouldn't be surprised if you were the only one in this thread who isn't using Dolby Headphone. If it means anything to you, the Mixamp sounds nothing like the CMSS3d you mentioned.