That can be a tricky thing to fix. Honestly you can do several things. Like I said before, try some messing around with voicings (height of your tongue when you play... thing vowel sounds), and definitely how open your throat is. Other than that, it really just takes time. Yamaha's can really have that thin hollow sound (at least at first), but in the end it just takes building up a sound inside your horn. I've heard some great players sound fantastic on Yamahas (both in the "legit" and jazz sides of playing), and I've heard people who's sound was paper thin. Try some stuff with voicings and throat shapes. It might not be a drastic change, and it most likely won't be an immediate fix. Just takes growing with the horn.

I apologize for the advice you hve received on this post. First of all, I think the mouthpieces you are using suck. If they work for you that is fine. The fact that you play a Yamaha Custom Z with a G1 neck and whatever mouthpiece you use has nothing to do with your issue. You say that your high C is in tune but it is a little "off". How easy is it for you to slur between the high C you speak of and the C an octave below? Can you trill between them with the register key? What about your low C? Is it in tune? Can you easily do a two octave jump betweem the two? I know that it is not easy but if you can it will help. The other thing you might try is recording yourself. Just because the note sounds a liitle "off" to you does not mean it is "off" to the listener sitting ten feet away. The high C (concert Bb on tenor and concert Eb on alto) is a difficult note. However, I have found that if it is in tune, the perception of it being "off" is in the ear of the player. Remeber, the way you hear your saxophone is not the way the audience hears it.