More eprops for the viola. Kim Kashkashian in particular is fantastic, almost everything she does she pulls off great. I'm a particular fan of the Elegies album, especially the Vaughan Williams and the Kodaly.
More excellent and relatively easy to get into chamber pieces:
Beethoven: Razumovsky quartets (Takacs Quartet, Alban Berg Quartet are both good)
Brahms: Piano Trios (Suk/Katchen/Starker or the Isaac Stern trio), Violin Sonatas (Suk/Katchen), basically almost all Brahms chamber music (there is an excellent box set by the Amadeus Quartet of Brahms chamber music)
Schubert: String quartets No. 13-15 (Alban Berg Quartet are heads and shoulders above the pack in these, imo)
Franck: Violin Sonata (many good ones, no particular favorite in my mind)
Rachmaninov: Cello Sonata (Isserlis/Hough is my favorite, but I haven't heard too many of these)