The class of tech is "DAC/amps" and here's 1 of the better (actually they make 2 differents units but they are similar--a walk-around unit and a desk-top). They work with all devices and OS's. Here are independent reviews: The E-10 http://www.headfonia.com/the-latest-...10-usb-dacamp/ and the E7: http://www.whathifi.com/fiio/e7/review Both have been significantly improved of late; consumer reviews on the Usual Suspect are ecstatic.
For something way different (and if you are bucks-up), consider the PonoPlayer. a DAC/amp AND a playing device all in one. Neil Young is a huge supporter and a Kickstarter crowdfunding campaign raised $2.5 Million in less than a week.: http://www.digitaltrends.com/music/n...ef-mp3-player/ << Interesting article but not a review per se.
NOTE: Even if the rest of this post has no relevance to you, know that Pono also runs a very high-resolution streaming and downloading services.: https://ponomusic.force.com/ << This may why Mr. Young got involved--so his music could be better heard by us sweating masses.
So what is a DAC and why is it important? Here's a sort-of answer (you'll have to wade through some tech jargon but a couple of answers are mish-mosh-free.): http://www.head-fi.org/t/201322/what-is-a-dac