The 500M is no longer available....but here goes. The 100S has better high frequency extension, depth, imaging, air and openness....the 500M has better bass, bass weight (maybe too full?), slam and power. I prefer the 100S. I have a new mod to the 100M amp where I add some extra capacitance to the main supply. I have not listened to it but I am sure it would help the bass. Will have more to report later when I have more time to listen....lots of projects going now. By the way, the amp is sold.....actually about 4 times over already.