I've learned that like our gear here (mics and amps), head amps are best paired with headphones specifically. Depending on your budget and how passionate/interested you are about this, I'd consider looking over at head-fi.org if you haven't already. Those people are as passionate about their gear as we are about ours. So while this isn't an answer to your question, it hopefully is another tool that will help you get to your answer.
For my Etymotic ER-6s, I came to the conclusion that the pico DAC/amp was the best thing I could do for what I wanted (not necessarily the price range though), but there were 3 or 4 others in the running.