You've missed the point.
Music players don't need amplifiers. Headphones need amplifiers. When the question is something like, "does DX50 need an amplifier?" then the answer is always "no". It does not.
When the question is something like, "do my headphones need an amplifier?" then the answer falls under what I've written: either they do for some technical reason or they don't. Everything else is stack envy. This includes wanting -- that is, the desire rather than the requirement -- some perceived improvement in sound quality which may or may not actually exist for a given set of headphones and ears and brain and listening environment, and especially when an inexpensive widget will do just as well.
Simple as that.
I think the person asking the question wasn't the most clear, and you didn't pick up on the implied question. Obviously, one doesn't NEED an external amp to listen to headphones out of any DAP, unless it does not have a headphone out. I do believe, what was meant by the question, is that will there be a sonic benefit by using an external amp versus the built in headphone amp. My reply to that is yes, if you own a high quality external amplifier, in most cases it will produce a (subjectively, by myself and many others) better sound than when using the built-in headphone amp. This goes both with IEM's and full size cans. My SE846's sound better from my RSA amps than they do from my DAP's headphone out. Now, how much better the sound gets, once again, is subjective, but it's enough of a difference for me to stack my shadow and dx50 when moving about. Everyone's experience will be different, and your setup will only be as good as your weakest link. But the discussions around this forum are decidedly leaning towards extracting the best possible sound out of the gear we posses (and will possess; just admit it, no one is ever done buying gear), is it not?