The 49990 is more detailed and also has better bass. There is a mod that you need to try with it. You need to put a Silmic 10uf capacitor onto its power pins. The easiest way is to use another socket. The Silmic must have 100 hours burn in. It sounds fantastic for the first 2 hours, then sounds kinda terrible for 20 - 60 and finally is settled at 100 hours. This mod is worth it with the 49990.