The
FiiO E5 is a very popular choice, due to it's small size, sensible price and warm sound signature.
If you are using the headphones with a computer a combined headphone amplifier and USB DAC may be worth considering. Take a look at the
iBasso range for some ideas. Most computer soundcards have very poor quality headphone outputs and DACs, so this solution can give a significant upgrade to sound quality.
Check out some reviews of the iBasso D2 below:
iBasso D2 Review on TrustedReviews.com
iBasso D2 Review on CNET