[haloxt]

If you can't get the 680 I'd say get the HFI-2200, it should be used with an amp but luckily it doesn't take a really expensive source to sound at its best because it's not detailed headphones at all, but it's about a million times more fun sounding than my Pro 900. Make sure it's the newer design with the aluminum faceplate and shorter 3m detachable cord with "HFI-2200 ULE" written on it, looks nicer than the previous version.

 

[drader]

The 680 is NOT more sensative than the 780... they both have the same gold driver! And both are as sensative as it gets. From your list I'd say go for the 780 - it is the newest of Ultrasone's hfi designs and well worth it. The hfi- 650 680 and 700 are nearly identical. The 500s use a different driver: better for bass, but less detail.

My coworker here was just trying to sell his hfi-650 with moon audio cable on ebay for 170. No takers. They are brand new. I have a pair of proline 650 with Jena Labs 18g cable. They are breaking in. I *may* sell them depending on how I like them this weekend. The Proline series has a "flat EQ" sound, whereaas the HFI series has a "U EQ" sound, which I prefer. You may also want to search for the Alienware Ozma 5 and 7. The Ozma 5 is really a hfi-550 (and also a DJ1), and the Ozma 7 is really a hfi-700. Not too many people know this, so the Ozmas go much cheaper than the Ultrasones!

 

[haloxt]

The 680 has a sensitivity of 99, 780 of 96, 2200 and 650 of 94.