If you're looking for warmth, the AMB Mini3 is a very warm sounding portable amp which isn't very difficult to build. I'd recommend tubes as well for the warmth you are looking for, but your Ultrasones have a low impedance of 40ohms so you'll really need an amp coupled with output transformers. Output transformers add significantly to the cost over OTL designs.
Some projects to look into:
Bottlehead Smack (not the Crack, only suitable for high impedance cans)
I hesitate to recommend the Torpedo as I haven't finished my build yet and haven't been able to listen to it with low impedance cans, but it's the lowest cost build of the bunch at under $300 and TomB and Dsavitsk have put an immense amount of effort into it.
Also note that nearly all tube amps are high voltage, so the utmost care needs to be administered when building any of them.
Edited by HiGHFLYiN9 - 1/11/12 at 9:35am