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The link you gave there isn't for anything to do with Mahler 2, but I'll assume that was just a copy and paste mistake.

You're referring to the brass choral in the 5th movement and what you're hearing isn't tuba down the octave, but a contrabassoon.

Cheers,
-K

If played well, you don't hear the contrabassoon - it just enhances the the tuba tone and is not independently audible.

There is a similar effect in Suk symphonic poem, Praga .