This was from another post and a different subject but applies to your issue in general. I agree that you should have a mouthpiece that works for you but practice truly does wonders!!!Roger Lewis said,
Rather than changing mouthpieces, spend some time practicing in the low register. Once your brain figures out the trick of getting the lips INTO the mouthpiece and soft tissue vibrating against soft tissue with a broader air stream, not a faster one, you will see the improvement you are seeking.
Mouthpieces are a compromise. You always have to give something up to get something else. In my limited experience I find that the horn makes about a 20% difference in sound and the mouthpiece makes about a 5% difference in sound. The rest of it is the human body. Whent you throw a tuba at a wall, when it hits, you will hear the only sound that IT makes. The human body is the instrument, the horn is an amplifier and changes the color of what you put into it.
Work on YOU first - you'll be glad you did.
Goodluck, Tim


