I'm using Clementine 1.4 on the latest Ubuntu Desktop.
I have the digitial output go via a Musicial Fidelity DAC with Digital Pre-Amp (DPA) Mode turned off and feeding into a Yamaha Amp.
I found that if I turn the volume on the Clementine player higher than 50, the output sometimes gets distorted. The distortian is most audible on something like a Ballad where the 's' sounds more and more like a hiss the higher you turn the Clementine Volume.
All my Music is in Flac.
Not sure which libraries Clementine uses under the hood to play the audio, but a friend of mine is a sound engineer and he can't find a reason why the Volume on the player should have any impact on the sound quality.