It may be 24 bit, but if the original source is bad, then no fancy converter will help much. Then may not have put any effort into the remix also, it was only remixed on the quick for marketing purposes. I thought from the title you were looking for the best recording made by Mr. Gershwin. There is a very nice modern recording of the entire Porgy & Bess conducted by Simon Rattle. Excellent cast of soloists I also have an old Leonard Berstein recordings of Rhapsody in Blue with Bernstein at the piano that I enjoy. Sonically not like a brand new recording, but an excellent performance and very good sound for the 1960s. I have the Columbia LP, but it is available in CD which may be better (Columbia Symphony - Bernstein Century CD, Sony Classics #63086) I have not heard it but the Bernstein Century CDs I do have are sonically excellent for a historical remix. This recording is also available in SACD, which gives you an idea of how well regarded the performance and the sonic quality is. (SACD is Sony Classics #89033) Also on the recording is Grofe's Grand Canyon Suite with now composer John Corigliano playing the violin. He composed the music for the movie "The Red Violin".
