Reading Simon Winder's excellent book Germania (about which I rave in more detail here), I stumbled across the surprising statement (on page 369):
... the vigour and pleasure of the pre-1914 world [...] can be summed up in the image of Mahler and Strauss in 1905 happily playing through the score of the latter's forthcoming opera Salome in a piano shop in Straßburg, ...
Which struck me as it falls in the period when both Heinrich the cellist and his then fiancée Maria were living in Strasbourg. Presumably they weren't so astronomically lucky to wander into that piano shop at the right time, but it made me wonder what Gustav Mahler (1860-1911) and Richard Strauss (1864-1949), then based in Vienna and Berlin, respectively, were doing in the faraway and musically less illustrious town.
As it turns out they were both guest conductors at the first Elsass-Lothringen Music festival, which aimed to build cultural bridges by featuring both French (Franck, Charpentier) and German composers (Beethoven, Brahms, Mozart, Wagner, and the two famous guests). Mahler conducted his 5th Symphony on May 21, 1905, and an all-Beethoven programme the next day (the Mahler Foundation provides full details for both days here and here). Assuming that Strauss also had a couple of concerts, and then a few for the French guest conductor, Camille Chevillard (1859-1923), this adds up to a whole week worth of music. The festival happened just one month after Strauss had completed the score for Salome, so at the time of that piano shop meeting, nobody in the world knew the music.
The whole event took place at the Sängerhaus in Sankt-Julianstraße, a huge venue built in 1903, which today is the Palais des Fetes in rue Selenick. Located in the Neustadt (German quarter), this location is literally only one block away from where Heinrich the cellist lived. So I'll henceforth assume that he did pop over to watch Mahler and/or Strauss do their thing.
PS: Here are some lovely postcards from the first decade of the Sängerhaus.
Stop press: I found the shop! It's Wolf Musique, it even had a plaque commemorating the one-man premiere of Salome, but it sadly closed down in June 2020 after 195 years in business. That news item was in French, but here's one in English. The French source mentions Mahler and his young wife Alma (along with baffled clients of the shop) as the audience, whereas Winder had only mentioned Gustav and left me with the impression that he and Strauss had played the orchestral score four-handed. The shop was located on the main island, 24 Rue de la Mésange.