Thing is, for most of us, it is difficult if not impossible to listen to different headphone amps before committing to buying one. Where I live (Montreal, QC Canada), there is 1 hifi audio store in the province that I know of, and their selection of amps is quite limited.
This is why we have to rely on second hand experience from users in this forum! 
If someone else had the chance to try the same model of headphones as we have, but with different amps, he can relate his experience and describe the difference that he heard. It's either that or reading the amp's specs... but that can be daunting, particularly for newbies (like me)...