What can I do to upgrade my system? Thanks

My current system right now is used for movies and music hooked up from my computer via optical cable from sound card built in to my motherboard. I listen to all genres of music and all different quality files from 256-lossless
I would say its 75% music and 25% movies on this system.

I was hoping someone could help me out with upgrading. What would be the cheapest upgrade that would give me noticeable gains in sound quality.

Here's my current system

Surround Front - Monitor Audio Silver Rx1's
Front Center - Monitor Audio BX centre
Rear Surround - Definitive Technology ProMonitor 800
Sub - Sony sa-wm250
Reciever/AMP - Denon AVR - 3805

My budget for upgrading is $400

I posted this on another forum and someone suggested I invest in a DAC(HRT music streamer pro) which started a debate in if a DAC would do anything since my Denon has a built in one. I figured this forum could help me out a little better. Also if a DAC wouldn't be my best option please let me know.
Thanks in advance