Sorry to resurrect this post. I've just bought a copy of this disc and heard it once through. I'm glad to report that this is without doubt a great Mahler 6th. Granitic, serious and trenchant, if Klemperer had done a Mahler 6th, I think it would have sounded like this. The orchestra plays terrific throughout (yes, minor slips in the brass; no, nothing major and certainly forgivable in a one-take live performance), and the percussion is fantastic (the tam-tam player is so loud in the finale he almost covers the second hammer--he deserves a raise!). Speaking of the percussion, the rute in the middle of the finale (accompanying the furious fast string-led march) is replaced with a snare drum for some reason. Sound is crystal clear but extremely dry, which renders the string sound somewhat wiry.
About the tempo: besides the Andante Moderato, which is probably the fastest around at 12:12, the other movements are pretty normal in tempo, and the first movement is on the slow side. By taking the Andante Moderato at such a speed Barshai seems to emphasize this symphony's lineage from Beethoven and even Haydn or Mozart: very flowing and light, but never lacking in angst in the climax or bitter-sweetness in the phrasing. One of my favorite renditions of this movement.