I've got some questions about what exactly is a white-box tester, so here's the gist of it:
A white-box tester is someone with strong technical coding skills that is able to dig under the hood to discover bugs before they manifest (as in corner cases that are rare) and find root causes of bugs already encountered, as opposed to black-box testers that focus on the visible symptoms that players would face.
Both may or may not utilize advanced tools and creative thinking to help get their job done, and both help us produce a more solid game. The difference is in the approach and skill set needed.
Just for completeness, a black box QA in our team is different from public beta testers in that they focus more on finding bugs in the game, help with test planning, produce bug reports, and work as part of the official team to help narrow down root causes of bugs discovered.
Hope that clears it all up, but feel free to post or PM me any additional questions you may have.