I should like to recommend the String Quartets of Elizabeth Machonchy and Hindemith's Kammermusik pieces. And from earlier times, the music of Landini and the organ works of Sweelinck are both a significant improvement on some of the repetitious extracts that are frequently served up on R3.
I'll nominate a trio of Russians: Arensky, Taneyev and Weinberg. (Okay, Weinberg was born in Warsaw, but was only 20 when he moved to the USSR at the outbreak of the Second World War and then spent the rest of his life there.)