The best word I can use to describe the difference is texture. There is an improvement in detail, soundstage and the overall oomph of the player, but the most striking thing for me is the texture of the sounds produced. Put simply I've never experienced this with a portable player before this, or even through my laptop via a DAC. Drums sound incredible, you can almost feel the impact. Guitars sound full, rich and detailed. Strings are airy and fill the room, and vocals are taken to a whole new level. The timbre of a voice is suddenly apparent, it's intimate, you really feel like the singer is right next to you. Put simply, every sound has a texture, you can almost feel it, and it is what makes this player stand out.
The sound is neutral, bass has impact and depth without being overwhelming. Mids are detailed, textured and absorbing, and the high end is crisp and clear.
The UI is where the main drawbacks are. Turning the player on takes 20 seconds or so but the overall appearance is nice enough, but with a music library of around 4000 songs, navigating is a chore. Going from top to bottom of my albums takes a minute or so of patient scrolling. I'd like to see iRiver add an option to skip to first letters easily so you can scroll through the list much easier. I've also experienced a couple of issues with metadata. Some albums aren't recognised properly and are sorted with other albums with completely different names, despite changing the info in MusicBrainz. I'd also like to see an update library feature because at the moment, if you add 1 new song and want to see it in your database, you have to update the entire library, and rescan every song which can take 5 or more minutes. Finally, I'd like an option to turn off the auto-fade when skipping songs, especially when it fades songs in sometimes.
The only other issue I have is that there is some EMI. Not acceptable for a high end DAP.
I use the AK120 with my Unique Melody reshelled Shure SE530x4 and they pair together beautifully. It also drives my Sennheiser HD-25-1 very well. I've heard that with 300 ohm monsters like the HD600 need to be amped with the AK120, but it's more than capable of driving most cans.
Overall, I'm delighted with the AK120, but it isn't perfect and for the price I'd like to see iRiver address some of the UI issues I've mentioned.