Yeah lots of people bash Labrie for being a bad singer. He has his screw ups, but is that not normal for someone to be singing at his range during a 3 hour concert for each concert date of one leg of a tour?!
I have only about 10 DT boots, Labrie is definitly a better singer now then when he was 10 years ago. Sure he could hit the extremely high parts of "Learning To Live" in 93', but his voice sound like crap when he wasn't pushing it. But!, during the 6DOIT tour, his vocals have since improved. Having seen DT live myself once, it was powerful to hear him and the rest of the band pull off the music that they create.
As for Russels live vocals, he is really good too. Im not too knowlegable about Symphony X's history and band changes, but I think he was singing on the "The Edge Of Sanity" bootleg (Osaka, Japan, 06/23/98) and he isn't perfect either. Although, i saw a video of SX doing "Egypt" at Progpower, and his vox's were friggin godly!. Both talented singers, imo