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I'm not overly familiar with the Jupiter 378, and I don't know anything about how you play, how long you have been playing, etc. However, knowing what i do know, I would think that a Conn Helleberg might be a good mouthpiece to look into. There are certainly many other possibilities. Possibly even a PT-36.

Perhaps you should give us a little more information. The best thing for you to do is probably to ask your teacher.