As references, over the years Willem has performed for:
- Ian Sadler, freelance organist/choir director
- Simon Irving, Conductor for Guelph Symphony Orchestra
- Roberto de Clara, Conductor for Oakville Symphony Orchestra
- Boris Brott, Conductor for National Youth Orchestra
- Uwe Lieflander, Conductor for World Youth Day Orchestra and Choir
- Anton Kuerti, freelance pianist
- Noel Edison, Director of Mendelssohn Choir and Elora Festival Singers.