This is what Gunther Joppig ("The Oboe and the Bassoon", Amadeus Press, Portland OR 1988) has to say about the Heckelphone:
Heckel, whose improved double bassoon had earned the approval of Richard Wagner, stated in his memoirs that the idea of the heckelphone in fact emanated from the latter. However, I have been unable to find corroborative evidence for this in the letters and diaries I have consulted. Heckel was a keen collector of musical instruments and was in touch with other collectors. Towards the end of the nineteenth century, while he was making reproductions of original instruments for them, he also produced a copy of a Swiss basset oboe. The heckelphone in fact has certain features in common with the basset oboe, in particular the very wide bore, and thus the idea that Wilhelm Heckel use the latter as the starting point for the design of his new instrument is not as far-fetched as it may seem. Of course the modern keywork meant that this was a completely new instrument. It was soon being played with great enthusiasm at various music festivals.
Richard Strauss and Max von Schillings (1868-1933) in particular made use of the instrument in their opulent scores, above all in their operatic works. In fact Salome (1905) and Elektra (1909) have kept the instrument alive. Richard Strauss' ballet Josephsgende, which for many years was considered unperformable, has only recently been convincingly staged. It also contains a part for the heckelphone, as do two other works by him - the Alpensinfonie and the Festliches Pr?ludium, an occasional work written in 1913 for the opening of the Konzerthaus in Vienna.
On account of its voluminous sound the heckelphone is particularly suited, it seems, to the orchestral palette of the late romantic orchestra; at least it was almost exclusively employed by composers with a penchant for this style. The instrument appears neither in the works of the Second Viennese School, nor in those of composers born between 1910 and 1920. An exception to this rule is Edgar Varse, who wrote for it in two of his works, Arcana and Am?riques.
A trio by Paul Hindemith (1895-1963) for viola (which the composer himself played extremely well), heckelphone and piano is seldom played on account of the rarity of the heckelphone. It is one of Hindemith's best chamber music works. He was probably inspired to write it on one of his visits to the firm of Heckel, which are documented by entries in the visitor's book. (Hindemith also acquired a bassoon for his own small collection.)
Hans-Werner Henze, Giuseppe Sinopoli, Hans-Joachim Hespos and Werner Schulze are among the younger composers who have looked favorably on the instrument and have written obbligato passages for it in their orchestral works. Continuing the late romantic tradition of the instrument, the Viennese composer Raimund Weissensteiner has composed a Sonata for Heckelphone and Piano. Recently the French have once again tried to revive the baritone oboe, and, since 1979 it has been made in France by Strasser-Marigaux, and also by Lorée and Rigautat. Those readers who are interested in the history and the repertoire of these fascinating baritone instruments are referred to the author's more comprehensive and detailed account, Die Geschichte der Doppelrohrblattinstrumente von 1850 bis heute und ihre Verwendung in Orchester- and Kammermusik (Verlag Das Musikinstrument, Frankfurt, 1980).