The 508 has better bass slam, wider soundstage and increased resolution. The 508 is for those systems that are clearly high end. The 508 has much better jitter control than the 506. The 506 is very good and competes with most in the price range.The 506 came out in 16 bit,20 bit and 24 bit like the 508.Each version is a bit better. How do they compare with lower Sony units.. no comparison. With high end Sony..again it is all system dependent. Let your ears be the judge!