Sadly, I think all the ABC quad masters were destroyed in the UMG vault fire. Audio Fidelity tried to find the quad mix of So What when they put out (what ended up being) their stereo-only SACD of the title. It's possible that safety copies of some of the other ABC quad mixes exist elsewhere, ie for Q8 releases internationally (Canada, UK, Japan, etc.) but for unreleased mixes like So What I think the only copies were the ones in the now-incinerated US vault.
The mix for the one track on the Koss Perspectives sampler is credited to John Stronach, who also did the excellent quad mixes of Donald Byrd's Street Lady and Sly and the Family Stone's Small Talk. Given how good (and discrete) those mixes are, the fact that the one track on the Koss sampler ('Falling Down') decodes somewhat poorly makes me wonder if they shelved the quad mix of So What in favour of The Smoker You Drink, the Player You Get which despite being kind of a mess compared to the stereo version (omitting or burying key parts) gives you a pretty whizz-bang result through a QS decoder.
It's also possible that (presumably being done by Stronach at the Record Plant and not at ABC's in-house studios under their supervision) that the other tracks on the discrete quad mix either broke some kind of QS matrix encoding rules, or exposed some limitation of the encoding process rendering the album unsuitable for matrix quad release - something they probably wouldn't have noticed until months after the mix was done and too late for them to go back and do it again, if that was the case. It just seems weird to me that ABC would be averse to putting out two Joe Walsh albums in quad in less than a year when they did exactly that with Three Dog Night (Hard Labor, Comin' Down Your Way) and Rufus (Rufusized, Rags to Rufus) right around the same time.
Like Jon and Mark, I concur that whatever circulates as a supposed matrix decode of the unreleased mix of So What isn't what it purports to be - if it was, the mix would match the Koss Perspectives version of Falling Down and not the stereo version.