TubeNet

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.

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!!!

Goodluck, Tim

I agree totally. But there is also the one non-variable: your jaw & teeth makeup. After you get your embouchure back reasonably well to where you can center tones and even a few lip slurs without warbling too band, try to borrow a couple of different diameters to see which one "sits" on your mouth better, and you don't feel like you're always having to make an adjustment to play. Since you have a Kellyburg, a Kelly 18 is just a bit narrower without being really any shallower, and there are others that are wider.

I was lucky in that I got to break back in after 20 years with a lodge band which had three sousaphones and a few oddballs, so I got to try everything from really shallow to really deep, and from a small King 25 to a DEG wider but shallower than the Kellyburg. I settled on a Wick 1 for my Besson and a Kelly 18 for my sousaphone.

Yes, everything is a tradeoff. For the Besson tone, I have to be really warmed up or I will crack all over the place. For the Kelly, I had to wrap the shank at the bowl with a piece of golfer's lead tape to improve centering, but oh, the projection outdoors!

I don't see how remote mouthpiece can be given. However, I'll jump in: Why don't you try an original Conn Helleberg too?