Erbtuba

I use the school tuba, it looks like it was dropped from an airplane. The tone is not clear, instead it was rather raspy. On the other tuba I use, the tone is super clear, and beautiful. On the school tuba I use a Yamaha standard 67 mouthpiece. On the other tuba I use a Bach 22. Yamaha compares the 67 to a Bach 22. So I do not know if the tuba is the problem, or the mouthpiece. I want to know because if the mouthpiece is the problem, I will purchase a new one. Please help!