you want to look at the number of outputs and type of outputs. for instance do you need a spdif out? coax? do you need 5.1? 7.1? or if you have a really nice pair of studio headphones with a high power draw you might want one meant for them. such models are typically 2.0 only with a big 1/4 jack on the back. an alternative to that would be an external DAC
you also want to look at what software it includes such as for virtual surround sound (if you use a stereo headset this is handy)
sample rates and resolution (the higher the better). higher = better quality.
thd (total harmonic distortion) the lower the better. basically how much the amp distorts sound.