Sharp 4th Partial Help?

[Womp$]

I just purchased a beautiful Conn 52j from Dillon Music, and after a day of playing it I've been noticing a few quirks. There's one thing that I'm hoping someone can help me with.

All but one of the quirks I've found I can work with, with no big issue, but...
I will tune to the low C and the bottom line G, and it will be perfect, but the C in the staff is super sharp. I can get all the other partials locked in with little to no effort, but that 4th partial is always about 20-30 cents sharp.

I'm wondering if it's me, my mouthpiece (Helleberg 120s), or the tuba itself. I haven't had this problem on other horns I've played, and I'm hoping it's a mouthpiece issue, but I'd love any advice or help anyone can give.

[Matt Walters]

For the conn 5xJ series of CC tubas, tune the C that is 2nd space in the staff. Then the C below the staff can be played in tune if you don't power open you jaw as if you were playing another 4th or 5th lower. i.e.
*There is a subtle slot that is in-tune that a relaxed player will naturally use.
*The less experienced player will open their chops too much and drop into a deep slot that is about 40 cents flat.
Tune the 2nd space C and then just play the C below the staff without thinking it is a low note. Don't try so hard.

E in the staff has to be played 1&2 because they didn't listen to my instructions or copy the 3rd branch of the prototype I did.
A on the top line is best played 3rd valve.

[iiipopes]

For the conn 5xJ series of CC tubas, tune the C that is 2nd space in the staff. Then the C below the staff can be played in tune if you don't power open you jaw as if you were playing another 4th or 5th lower. i.e.
*There is a subtle slot that is in-tune that a relaxed player will naturally use.
*The less experienced player will open their chops too much and drop into a deep slot that is about 40 cents flat.
Tune the 2nd space C and then just play the C below the staff without thinking it is a low note. Don't try so hard.

E in the staff has to be played 1&2 because they didn't listen to my instructions or copy the 3rd branch of the prototype I did.
A on the top line is best played 3rd valve.

This. ^ From the guy who designed the tuba.