My setup like this: onkyo HT-R340 5.1 receiver, wharfadale diamond 9.6 and media pc, i prefer listening music mostly time, i dont have too much bugdet, what i can do for better stereo performance, also i have xmeridian 2G sound card with LME49720HA opamp and connected multichannel to the receiver.
Your setup is pretty good already. I wouldn't make a lot of changes, maybe you can try doing some software EQ and VST plugins (changes in soundstage, depth etc) to make your music sound more realistic. That'll produce the biggest change, more than any dac/amp.
Have you heard about the TB Isone? http://www.toneboosters.com/tb-isone/
Works with Foobar with the VST adapter: http://wiki.hydrogenaudio.org/index.php?title=Foobar2000:Components/VST_adapter
Also give some thought to room acoustics, they play an important role.