For recording live concerts, in the past I recorded off of the musicians master line out from their soundboard/mixer to a mix pre and then going into my camera. However, I often got bad mixes where the guitar was not loud enough compared to the voices or vice versa. I will be getting a 788T Sound Devices 8 channel recorder and was wandering if XLR Y cable spliters for each microphone on stage before it goes into thier sound board for each channel would be a practical solution to trying to use my own microphones to double record them. They would be the XLR F to two XLR male Y cables. One male would go into the their sound board and the other male for each microphone on stage (6 for example) would connect to a longer XLR cable that eventually goes into my 788T. Would this degrade the sound for both the musicians on stage and for the split lines going into the 788T as line in's? I want to get a record of each channel of vocal and guitar for all three musicians on stage and then later remix them to the desired levels. Would a snake box be more what I am looking in order to not degrade the quality or would 6 simple 20 dollar y cables do the trick?

The problem is that mic'ing and mixing for live sound are totally different from mic'ing and mixing for recording. Indeed, you might not even have mic's on all the performers - guitar cabinets and drum kits are often loud enough to carry into the room without the need for additional amplification through the PA. So no mics means your recording of them is dependent on the bleed into the vocal performer's mics. Picking up the sound off of a DJ's board at a wedding makes sense cause he's playing a recording and everything is going through the board. But a concert venue is something else and you might want to consider mic'ing the band yourself with your own totally separate system, perhaps mic'ing the stage with a stereo pair from the front of the audience as if it was an acoustic performance.

I would be reluctant to try it with simple "Y" cables. A mic splitter would be a better bet if you want to try that approach.

Well what kind of mixer is it? Most, if not all live sound boards have direct outs on each channel for each mic. You probably don't even need Y-cables at all. Just the cables to patch from the board to the 788T.