Has anybody heard the multichannel mix yet? I am curious if it is a conservative mix, or if it plays instruments in the surround channels like the Herrman disc or Bartok concerto for orchestra by Boulez. And how is the recording quality in general?

I have compared the Dutton SACD with my original 1978 CD (RCD13650).
The Dutton disc is much superior overall - somewhat brighter and lighter in the bass.
The Dutton quadraphonic mix from the original multitrack tapes is, I would say, quite conservative compared with those you mention.
Hope this helps.

Thank you. Meanwhile I have ordered the disc. It will arrive soon and I will post my impressions.

I have listened to this disc now, and I do not agree that the MC mix is conservative. Rather the orchestra is arranged in a circle around the listener in Tacet-style. The strings are between the fronts, all the other instruments form a big circle through the surrounds. For example are the trumpets near the right surround, and the French horns at the left surround speaker. It is very impressive and allows to follow the dense orchestration better; I like it very much. Else, sound quality is fine for a recording from this time. A bit weak in the bass, but Double basses, tuba and big drum are still noticeable. Hiss is negliable, no offensive distortions, a rather clear and clean sound. The music is a great selection from 2 important scores from Williams, both from 1977. They show off his broad stylistic spectrum and imaginative orchestration. Very recommended.

If you have set up your system to give those results, then that's fine.
However this is the usual conservative surround mix.
It was never recorded or released in surround and Mr Dutton has only opened up the soundstage with his Quad transfer.
It is the best that this recording has ever sounded and is highly recommended.
A recording that is "very surround" and was originally recorded that way is Carmina Burana on Dutton Epoch 2CDLX369.
Give it a try, its great.

This has nothing to do with my setup. MC recordings with ambience only in the rear do just that. While this recording has clearly discreet instrument placing in the surround channels. Thanks by the way for the Orff recommendation, I will check it out.