The low register can get flat and "flappy" if the jaw starts dropping and the chops sort of spread and seperate. I try to keep the chops forward (instead of dropping) into the low register to keep the lips together. If the third space C works well, you might try starting on that C, than go down chromatically to the low C. Then try going down the major scale, then arpeggio, and try to maintain the same sound. Good luck! Jason.