What’s wrong with Tchaikvosky’s Third Quartet? It’s so morose! Akin to his Piano Trio. Have you heard his String Sextet (Souvenir De Florence)? It’s wonderful.
The Piano Trio in A minor is a wonderful piece. But a few years ago, I had a chat with a cellist who appears regularly at our summer chamber music festival in Seattle. We were comparing it to the Shostakovich Piano Trio in E minor. He said, "After hearing the Shostakovitch, you want to go home and slit your wrists. After the Tchaikovsky, you want to go home, pour a stiff drink and forget about it." He had a point. Shostakovich cuts closer to the bone.
The String Quartet in E-flat minor was an ordeal to listen to. Morose or not, it doesn't compare to the first quartet in D Major, which is a classically composed gem. A composer with Brahms' critical sense would have suppressed that piece.