The 5.1 is a brand new remix done from the original multis by Elliot Scheiner, specifically for the DVD-A release. The quad mix (released on CD-4 vinyl, Q8 tape, and Q4 tape) was done by Don Landee & Ted Templeman. There are some similarities in terms of placement to the quad, but it's definitely not the same.
Here's a few differences on the track "China Grove":
The 5.1 has the two electric guitars in each front channel, and two different piano parts in each rear channel. The piano parts in the rears are buried way down, to the point of being nearly inaudible, on both the quad mix and the stereo mix. The end guitar solo is in the phantom center.
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The quad mix has one electric guitar part in right front, and the other in left rear. What's interesting about the quad is that the right rear channel is nearly empty for most of the song. There is just some light tambourine until the very end, when the lead guitar solo bursts in from that channel only.
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Both have the backing vocals ("talkin' bout the china grove...) isolated in the rears.
I like 'em both. I think Scheiner's 5.1 is ultimately the better mixing job, but the quad can be much more aggressive at times. You won't see lead guitar coming from one rear channel in a modern 5.1 mix.