Well, I've heard things about the Jupiters like what I have about the cannonballs. They claim to be pro quality but dont really live up to the standard. But then again, I don't really think the horn could make you squeak, but it could make he bad response issues you're having. But it might be all the mouthpiece lig reed setup. What do you use as far as that? You may need to change reed sizes. Also, something a proffesional friend told me a long time ago that I've always found true, is check the chamber in your mouthpiece. Look in it, and if that opening is square, get rid of it. A square chamber does not belong on a circular neck. Saxman or whoever....don't jump me for this and call it rediculous, it came from someone who knows a lot more about what they are talking about than you.

I happen to have a new Jupiter 4C plastic mouthpiece in the 'ol shoebox, and I'll say it has more of a tendency to squeak than most other mouthpieces I've played. A cheap alternative that plays with similar tone to the Jupiter stock mouthpiece (but with much less squeaking) is the Yamaha 4C. The Jupiter I've got has a round chamber - like they were going after a Larry Teal design, but the walls are quite bowed and it's just kind of a unique design. On the brighter side, which is why they can tend to be squeaky unless they get worked on and adjusted a little bit.