Looking to spend around $300 but can be extended to $400 if absolutely necessary. I'm enjoying the sound signature of the Shure's in the sense that it doesn't excel in an area but is overall a very balanced headphone since I listen to pretty much all types of music. I'm fine with open or closed...

I currently have the Shure srh840's with an Asus Xonar DX listening to music through winamp w/ kernel streaming plugin.  Now my question is what do I need to upgrade next?  I had some suggestions about an amp / dac but I don't really understand what that means.  I'm looking to spend somewhere...