I trim it just by listening to the sound. Therefore I play a low volume audio part and adjust the trimmer to minimum audible noise
Fair enough.
Of course, that DAC never reaches true 16 bit
Yes that's right.
There are two main causes of noise in a digital audio system, one is quantization noise of the amplitude of the signal, and the other is the noise introduced by the quantization of time or sampling period. By over sampling you can cut down the latter but the former is down to the number of bits on the D/A I got very good results in the 70s with 13 bit converters, which in those days cost the same as three months research grant.
The best titled academic paper I read during my lit search was:-
The effects of alcohol on perceived quantization noise.
Which surprise surprise concluded that the more alcohol you gave some on the less they noticed quantization noise.