Re: Upper tone quality of the Yahama Custom Z Tenor
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.