If you're set on an 85mm prime, I'd actually suggest the 85mm f/1.8 stopped down to f/2.8-4. Fact of the matter is it's the fastest and most accurately-focusing 85mm prime for Canon bodies (and I say that even as someone who regularly sings the praises of Tamron's newer AF systems) and at those apertures, all these 85s end up being more-or-less the same optically, though the Canon f/1.8 does continue to have less aberration than any other 85 available for Canon. The new 85mm f/1.4 IS is a touch faster to focus on some bodies, but your 6D isn't one of those, and the optical improvements (which are only center sharpness and vignetting) don't mean much if you're stopping down anyway. (The IS meaning even less at the shutter speeds you'll usually be using for a performance.) For the sake of that lower-light concert photography, where AF really matters a lot, it's very hard to recommend the Canon 1.2 (vague focus-by-wire), the Sigma (inconsistent accuracy), the Tamron (slower), or the Zeiss (even for jazz concerts, you don't want to be trying to use a Zeiss; I happily use them in the studio, but they absolutely do not hold up for concert shooting).
Slightly better than that would be the 100mm f/2 and 100m f/2.8L IS Macro. The plain 100mm f/2 is one of Canon's best lenses for controlling aberration (as far as primes go it's a tie with the 85mm f/1.8, which, again beats every other 85 available in Canon mount) and it's also the fastest-focusing lens over 50mm that Canon produces. The f/2.8L is slower to focus, as it's a macro lens with a much larger focus throw, but it's optically even better and gives you IS and weather sealing.
But really, better than any of the above would be the 70-200 f/2.8L IS II. It focuses faster and more consistently accurately than any of the primes except for the cheap 85 f/1.8 & 100 f/2; it's optically as good as you can get (the only Canon-mount lens in the world with less prominent aberration is the Zeiss 135mm); and there will absolutely be times you are thankful for the extra range. There is a reason why the 70-200 2.8 is the performance lens, whether it's an almost motionless piano recital, ballet, theatre, or a heavy metal festival.
If you're absolutely dead-set on primes and 85mm, the Canon 85mm f/1.8 (or slightly better, the 100mm f/2) shot at f/2.8 or smaller is equal-or-better than everything else in terms of both AF and optical quality. If you really want to shoot wider than f/2.8 then that lens does drop behind the others quicker, but for jazz band you should have more than enough light for f/2.8 and I'd never risk missing focus by using a wider aperture than that anyway.
If you can't bring yourself to use a non-premium lens for whatever reason, the new 85mm f/1.4L IS is the second-best choice on paper for this kind of purpose (I've yet to use it enough myself to definitely say how it fares in terms of aberration, but so far it seems fine). I love the recent Tamrons, but the AF is a touch slow and if you're going to give up AF speed then you might as well get the Canon 100mm f/2.8L IS; I like the recent Sigmas, but their AF—especially on the 85—is too inconsistent for me to trust on an unrepeatable task like any kind of concert. The Zeiss, as I said above, I love in the studio, but I'd never take to a stage performance.
But, like I said, the 70-200 f/2.8 IS II is king of them all. You'd have to be really, really against zooms to use anything else.
