If you sound good over middle c and bad under low c. AND you sound good when playing loud but bad when playing soft then your chops are TOO open.
The aperture adjusts for both range and volume. Work on long tones include long crescendos and decrescendos. You know the stuff at the front of the Schlossberg. That teaches aperture control.
I also like lip buzzing.
Pops

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Pops