One of the cool things about Spotify, the streaming music service recently transplanted from Europe to the U.S., is that its subscription service allows users to play unlimited streaming tracks from mobile devices. With a recent update, the iOS version of Spotify’s app has doubled the quality of songs you can play when using your iPhone, iPod or iPad to access the app.

Spotify works by allowing users to stream music both that they own and from Spotify’s online database of thousands of tracks. It’s somewhat like the personalized Internet radio services such as Pandora, except Spotify allows for on-demand searching for songs, bands and albums. It’s available both as a free service with ad support and for a monthly subscription fee of about $9 – but if you pay the monthly fee, you also get streaming to mobile devices.

Up until recently, the mobile app for Spotify supported only two quality levels for streaming: 96 kilobits per second and 160 kilobits per second. The lower of the two is pretty data-conservative – not bad for saving money on your smartphone’s data bills – but sacrifices quite a bit of fidelity in the quality of the track to which you’re listening. The 160kbps rate is better and sounds pretty good through most sets of headphones or speakers, but with better-quality equipment, users can pretty quickly start to hear the limits of the 160kbps transfer.

The update to Spotify’s app adds a third level to its streaming capabilities: “Extreme,” a transfer rate of 320kbps (the lower rates are referred to in the app as “Low Bandwidth” [96kbps] and “High Quality” [160kbps]). That’s double the former highest quality level the app could achieve, and tracks streamed at 320 kbps sound substantially better than their lower transfer rate counterparts.

Also available at the 320kbps quality is Spotify’s “sync” feature, which allows users to save some tracks for offline play as well, allowing them to be saved at the higher quality level for later. The app also allows one-touch log-ins through Facebook with the update, which means getting to your music with your Spotify account goes a lot more quickly.

It’s worth noting that the higher stream rate means Spotify will suck up significantly more data for users who fire it up on a 3G connection, and that can cost you on your monthly bill. If you’re lucky enough to still have an unlimited data plan, however, or if you use Spotify over a Wi-Fi connection, there’s nothing to worry about.