Obviously some stuffiness is caused by leaks, damage, supoptimal key heights, octave issues (particularly with upper A and C), etc. But what about when you’ve taken it to a competent tech and worked on it and nothing seems to be helping? Well, the following information may help you.
Some notes on the horn are vented by one or more open toneholes directly following, some notes are not. The notes that are not, like D, E, A, middle and upper C- have to deal with relatively less venting than those with more than one open tonehole directly following, and this can sometimes affect intonation or the relative stuffyness of the note.
Why, you ask?
A simple example- low E only has one open tonehole directly following it (the D tonehole, the Eb is closed, then the low C follows), and thus less venting. Great, so open up the D tonehole to give it more venting. Problem: now F is sharp. Therefore, compromise is needed. Through tonehole positioning and tonehole sizing, a reasonable compromise can usually be found, but sometimes not without auditory evidence. Like middle C. It sounds way stuffier than side C- hence the old-timers calling side C “ballad C”.
For a simple test that will illuminate this principle, play F and slowly lower your D key. The tonehole directly below F remains open and unchanged, yet the intonation and sound is affected.
Depending on you, your setup, your horn- results may vary.