In this paper, we address the problem of assessing the perceived quality of estimated source signals in the context of audio source separation. These signals involve different kinds of distortions depending on the considered separation algorithm, including distortion of the target source, interference from other sources or musical noise artifacts. A new MUSHRA-based subjective test protocol is proposed to assess the perceived quality with respect to each kind of distortion and collect the scores of 20 subjects over 80 sounds. Subsequently, the contribution of each type of distortion to the overall quality is analyzed. We propose a family of objective measures aiming to predict the subjective scores based on a decomposition of the estimation error into several distortion components. We conclude by discussing possible implications of this work in the field of 3D audio quality assessment.