For people who want to know how can he benefit from an headphone amp, I have some opinion by myself.
1, There are two kind of amp, one is designed to provide enough power output so the headphone can get enough power so you can get 100% SQ.
another kinds is designed to modify the SQ, for example, you have a " cool " headphone, so you want a " warm " amp to change the system to “warm ”， or you are Bass lover so you need a amp with
bass boost if the bass of your headphone is a little weak.
2, To make sure how big the different you will get by add an portable amp in your system, first you need to know what you have and what you are looking for.
for example, usually big cans need a headphone amp because almost all need much more power than a mp3 player or smartphone can provide, of course it can sound, but the SQ is not so good,
so it is easy to reply that if someone ask me when he has a headphone which with low sensitivity or high impedance because 99% person can hear the different between amp and without amp.
but is will be very difficult to me to reply when someone have special taste of the SQ, someone love heavy bass and some don't , some love Rock, some love Classic, so that is why there are so many kinds of hi-end brands,
So there will not have standard reply unless you know what kinds of sound you love and will you have enough listening experience to identify the subtile different in SQ.
3, Also remember everyone have an unique ear and change by ages, I had discussed with the Sound Engineer from Westone, they told me that there may have 10 - 20 dB listening different for some people in some special frequency,
so it is not strange that you may feel there are not any change when you buy am amp and pair with your headphone. so the best is read more review and feedback in head-fi, you will not be wrong if so many user said that they like
one special amp , and it will be great if you find they use the same earphone/headphone that you have.
LOL, it is quite long and not easy to me to explain my opinion, hope someone can help me explain it better!