Upmixers that overuse the center channel degrade the stereo music soundstage.

Evidence

Much criticism of upmixing is related to how the frontal stereo soundstage is treated. Some upmixers generated unflattering renderings of the music soundstage, especially if too much center channel was employed as happened in some movie-based algorithms. In my experiences, David Griesinger’s early versions of Lexicon Logic 7 with adjustable parameters were excellent for music upmixing, adding pleasant envelopment cues while minimally changing the stereo soundstage, but one parameter setting did not suit all recordings. However, the products containing those versions are long gone. Nothing since has been as satisfying, although I have found the Anthem upmixer and the Auro3D upmixer in some settings to be helpful without being offensive. — Toole, Floyd E, Sound reproduction- the acoustics and…, 864