Denon 1610 is a fantastic receiver for your need. It has more than enough to drive S520s. Have you run Audyssey setup yet (using the mic at 6 positions with a tripod)? It will make a huge difference in sound.
Try Audyssey, set your receiver to output full range sound to S520 if you don't have a subwoofer. Based on the specification Audyssey uses, it will likely to set your speakers to "Small" but you can manually change that to "Full-Range".
For Audyssey setup, there is a detail thread in www.myav.com.tw.
It takes some time to understand Dynamic EQ and Dynamic Volume. I'd suggest you turn off Dynamic Volume whne you're listening to music. You can try to turn off Dymanic EQ if you feel the bass is too strong. These are great elements for movies but depends on personal tastes for music.