Heya,
I have the Ultrasone Pro 900's. They do benefit from amplification. But they don't need much. You should try to decide between solid state and tube for amplification. I happen to like the sound of my Pro's on my Tube amp (the Little Dot MK III). The bass is rich, low, warm and tight. Sounds great.
Really though, for the money, the FiiO E9 is hands down going to deliver every bit you need, and more, for the money. I would suggest you go there if budget is needed.
Otherwise, I can't help but recommend a tube amp. It makes my Ultrasones sing so nicely (warm amp + cold ultrasones = great).
Very best,