I was recently listening to RVW's highly intriguing late
Violin Sonata (1952), in this excellent recording by Hugh Bean and David Parkhouse:

The first two movements represent RVW at his most "modern" and complex - full of irregular rhythms, unpredictable harmonies, and scintillating virtuosity for both instruments. Great stuff - so it comes as a slight disappointment to me that the variation-form third movement returns to a much more "comfortable", traditional modal idiom, but it's still beautiful music nonetheless. Interestingly, the theme is taken directly from the third movement of his early Piano Quintet (which is also a theme-and-variations) from the other end of his illustrious career.