That's for the MixPre-6 1/8" input. The XLR inputs can handle much stronger signals (+14 dBu mic-in and +40 dBu line-in)...
As for the 706, your AKGs (20 mV/Pa) will put out -8 dBu at 118 dBSPL. That's pretty loud, but definitely not out of the question for a rock concert. You would need to use attenuators to prevent overloading the inputs at that sort of volume level. You'd get an extra 20 dB using line-in, but if you're using a pre with fixed gain, or minimum gain, you would lose some or all of that again. Maybe you can use phantom in line mode, though (it is not clear to me from what I have read), which would help.