The diaries from 1926 and 1927 reveal something about Schenker's activities as a performer that I was not aware of. Around this time, he frequently played piano four-hand music with his wife Jeanette (Lie-Liechen), including transcriptions of Haydn symphonies and - unsurprisingly - the Brahms Liebeslieder waltzes; on other occasions he played four-hand music with his pupil Anthony van Hoboken, and with Otto Erich Deutsch.
Schenker also played (and worked) through some cello sonatas by Beethoven and Brahms with his nephew Georg, and played piano trios with Georg and his father (i.e. Schenker's brother, "Mozio"). That would make Mozio the violinist of the family. On another occasion Schenker and Mozio were joined by Theodor Baumgarten, the family solicitor, in an evening of piano trios; that would make Baumgarten a cellist.
Occasionally Schenker describes these activities as strenuous (anstrengend), because he is afraid that other company present might find this activity tiresome or distracting. But all in all they are testimony to someone who was - and remained - a practising musician.