I always wondered why the complete cycles were not recorded. The extant quartets by both composers are quite wonderful (in different ways). If others know more about availability, please let me know.
Respighi (only 3 of 7 available)
String Quartet in D, P. 18 (1898)
String Quartet in Bb, P. 20 (1898)
String Quartet in Db, P. 46 (1903)
String Quartet in D, P. 53 (1904) - available
String Quartet in d minor, p. 91 (1909) -available
String Quartet 'Dorico', P. 144 (1924) - available
String Quartet in D (in one movement), P. 190 (date?)
Rimsky-Korsakov (only one available)
String Quartet in F, Op. 12 (1875) -available
String Quartet on Russian themes, Op. 31 (1879) (only 4th movement 'in a monastery' available
String Quartet 'Jour de Fete', finale (1887)
String Quartet in G (1897)
Allegro in Bb (for Les vendredis) (1899) -available
I have a recording of Rimsky's G major which I will upload soon.
Thank you!