"I can focus the 6D's center point on a subject with reasonable contrast down until autoexposure gives me a setting of 160 at 10 seconds and f/2.8 (really dark) with the Canon EF 24-70mm f/2.8 L II USM Lens mounted (note that the 6D's metering range spec is listed at EV 1-20). The Canon EOS 1D X's center point could not focus on the same subject with the same lens mounted. Note that the 6D focuses very slowly under these dismal lighting conditions - but locking slowly is far better than failing to lock.
I'm having very good success with the Canon EOS 6D's center AF point. And this is again the AF point I am using most frequently on this camera. It properly and accurately locks onto subjects as I would expect. "