@allan
To be honest, I was a little confused myself, for the same reasons. She did tell me that someone could be excluded if their tinnitus couldn't be masked at a safe level. But when I was being tested and I could still hear it a bit on the third go, she seemed happy enough to stop there. I'm totally speculating here, but I believe it's about hearing the sounds, not masking your tinnitus. If your tinnitus is so loud that you can't hear certain sounds they play, then maybe that nullifies the treatment. I could still hear everything, but could also hear my tinnitus over the white noise, to some degree throughout the testing. White noise is a bad sound for me to mask with, if I use high pitched beeping noises or bird chirps, I can mask it at just a very low volume. I might point that out to her next time I'm there.
Mine is complicated by the fact that going louder doesn't mask my tinnitus, to a certain extent keeping it quiet does. My tinnitus is loudest in noisy environments, it rockets up above everything. Yet, I can make it go very low in a quiet room.