The field of mechatronic instrument design has developed substantially over the past decade. With these new instruments the common musical assessment techniques that are generally applied to western art music are no longer sufficient as the instrument builders need to assess the design and production of the mechatronic instrument itself as well as the way it might be used musically and expressively by composers and performers. This paper introduces ideas about the new approach to the assessment of mechatronic instruments that is needed to fully assess them. After introducing the new assessment technique principals and discussing why a holistic approach is needed in this particular field the authors then go on the provide a case study on the speaker.motion mechatronic loudspeaker system that has been assessed in the proposed way. The paper provides details on both a qualitative and quantitative assessment of the speaker.motion system and how the combination of the two studies is what allows a full assessment of the instrument, and of its expressive potential.