The clear, honest, open sound projects best in the low register.
Muffling the horn sound with the right hand is like covering your mouth with your hand when speaking in public. Low notes have long wavelengths (several feet long!) To let those wavelengths out, we have to keep the right hand flat and open with fingers and knuckles flush against the throat of the bell.
Another common pitfall is trying to "control" the low register by thickening the back of the tongue or constricting the pharynx. If you look at an anatomy chart, you will see that the tongue looks like it goes halfway down the throat. When we thicken the back of the tongue, the throat becomes constricted and the sound suffers. The tongue should be kept quiet and comfortable; you want to get a "yawny" feeling in the back of the mouth. (Try yawning and feel how cavernous and relaxed the inside of your mouth becomes.) Think of the sound as "forward" (the "oh" vowel will help you) and the air as flowing.
The following exercise is useful to keep the tongue quiet and the sound open and clear as you go into the low register. Sing with the syllables "thoh-hoh-hoh-hoh" as you go down the scale. Drop the jaw incrementally to progressively increase the size of the oral space as you descend. Transpose downward, half-step by half-step. Record yourself to get feedback about your sound quality.
Finger-breathing is also an effective cure for constricted sounds in the low register.