There's a big EMI Tennstedt box containing three of his live recordings as well as the studio cycle (live M5 from 1988, live M6 from 1991 and live M7 from 1993). I find them to be both interpretatively and sonically superior to the studio performances. Quite often his studio performance sound contrived (the plodding finale of his studio M5 for example), and at any rate they were ill-recorded by Tennstedt's unwilling producer John Willan who didn't like Mahler. Those live performances are mostly notably broader than the studio ones (even though the live M5 finale is much faster than the studio one), and they never fail to amaze me for their broad sweep, sensitivity, drama and spontaneity, as if the music was just composed at that moment.
The LPO plays almost perfectly during those live performances and sounds so much more powerful and translucent than the studio ones. (The brass section really flexes their muscles during the climaxes!!) I'd like to single out the 1991 Sixth, a positively terrifying reading with passion and darkness equaling--even surpassing--that of Lenny in his VPO recording for DG. The hammer blows are only second to Levi on Telarc in terms of sheer presence. A very broad performance, certainly (91 mins), but always devastatingly effective. Most delightfully, it does not sound deliberate at all. My absolute favorite Mahler 6, bar none.