When judging the "punch" of the transmitted audio using the 159L, are you listening to yourself with another nearby radio, or are you actually talking to someone far away, and they are telling you that your audio is low?
I have the 159L and the 158L on my demo radios and the ones I have sold to other customers, and have not heard of this complaint.
When I am using the 159L on one of my radios, I speak across the mic with my mouth about 2 inches away, and have been told by the user on the other end that I was coming across with a very good signal.
Steven