Yeah..... I am in the same boat. I miss Udo, yet I love Mark. If Mark quit and Udo came back, I would miss Mark.... both guys were simply meant to sing for Accept.
Mark doesn't sound "like" Udo, but he sounds like Accept....... if that makes sense. However, even though Mark sounds great covering many classic Accept tunes, it isn't the same.
Perhaps something to do with Stefan leaving the group and no longer producing his robot-tor style as he seems to have left to produce the Acceptology video of the band's career.
"Stalingrad" is not "quite" as good a BotN in my ears, but it is not that far off. The middle of the disc is incredible !! Revolution, Against The World and Twist of Fate are as good of a back to back to back trio of songs as any I can think of.