Just been listening to the 1956 Mravinsky mono on DGM 18334, from a British lp pressing. Like the 1960 Wembley Town Hall stereo sessions, an enormously impressive performance, totally unsentimental, and somehow unfolding with a kind of natural inevitability than can only be the product of a conductor exercising absolute control.
Well worth hearing, though I'd say my preference is for the 1960 version, in part because the sound's just a tad distant. Mikes at one end of the Musikverein, orchestra at the other – possibly because the engineers needed to accommodate the sheer ferocity of the brass section. There's a dark profundity here which reminds you it was only a decade since WWII, and here in essentially enemy territory the orchestra is representing its country's sufferings.
For that reason, other performances get more colour out of the music (I've a soft spot for the Sinopoli, notwithstanding all his pushing and pulling at the score), but for me Mravinsky is absolutely the starting point to get to grips with this symphony.