The complete recordings of the Amar-Hindemith quartet are available in a 2 CD set and not particularly expensive:
https://www.discogs.com/release/17898544-Amar-Hindemith-Quartet-Mozart-Beethoven-Dvořák-Verdi-Reger-Hindemith-Bartók-Křenek-Stravinsky-Co
Audio quality is "rough", but I grew up on Bird bootlegs, so hey...
What's striking, to refer to other reviews, is how direct this playing is. There's a lot to be said for that in these pieces! I don't know that anybody today would do it like this.
Of course there's some overlap with the Hindemith thread:
The original Amar Quartet was assembled out of necessity in 1921 at the first Donaueschingen chamber music performances in order to play the premiere of Hindemith's String Quartet, Op. 16. The sensational success of this premiere encouraged the musicians to continue performing together as a quartet. This was a stroke of luck for the contemporary art of the quartet, for the musicians intensively devoted themselves to the cultivation of modern chamber music, including Bartók's Opp. 7 and 17 and Schönberg's Opp. 7 and 10. The ensemble was named after its first violinist Licco Amar; the second violinist was Walter Caspar, Paul Hindemith played the viola, with the composer's brother Rudolf Hindemith or Maurits Frank on the cello part.