His research on Beethoven's symphonies started in 1984. In December 1996 the score for the Ninth Symphony was published, attracting attention from the press. There were four recordings of the symphony prior to the publication of the new edition, with the conductors using the Breitkopf edition with the corrections as they were available to them; Roy Goodman was the first conductor using corrections by Del Mar, followed by Sir Charles Mackerras in 1991, Sir John Eliot Gardiner in 1992, and Claudio Abbado in 1996.
The publication of the new editions of Beethoven's symphonies was concluded in 2000 with the Seventh Symphony. Other works he studied that resulted in new scores being published were the cello sonatas (2004), string quartets up to Op. 95 (2006-8),the Violin Concerto, as well as the Dvorak Cello Concerto. He is currently working on the 5 Beethoven Piano Concertos.