I'm surprised it took me so long to find this thread considering I've done nothing but research this headphones, which is still happening after purchasing and receiving them. I like them A LOT. This survey was great; I was considering a lot of different amps/DACs but after trying them with a few different DACs/amps already, I agree you can run them on anything. I think they scale up decently from a portable device.
One thing I did notice is that on a DJ mixer, I had to turn the headphone volume up higher than with my ATH-M50s. I'm not sure how their impedance differs, but just an observation I felt like sharing. I also noticed the X1s sound fine through a Yamaha receiver where as the M50s sounded like I was listening to a lower quality audio file.
Thinking I'll just stick with my setup and maybe get an SBZ for surround purposes.