When using the HiFiBerry DAC+ Pro, the output signal is much lower than my standalone DAC (and other devices) and seems of lower quality (probably just a bias because the other one is louder). When you go to alsamixer, you’ll notice that playback is at -6dB, 0%. According to the DAC manual (iqaudio.com/downloads/IQaudIO.pdf).
As for the PCM signal. There is no mentioning of dB or clipping in my case, but the default of 96% seems exactly correct. The playback gain I do not yet understand, but I think it should be left at 0%, the default setting of the DAC (page 99 of the pdf). I know IQaudIO recommends to put them all at 100%, but I’m not sure. At 96%, 100%, 0% the output is indistinguishable from my standalone DAC.
Note that people are already complaining because of this: rpi2-iqaudio-dac-plus-horrible-sound-t3791.html.