you dont need an amp, your sources are already powerful enough to amp those sets as they dont need much power. also they aren't exactly amazing sets of headphones either. rather than buying an amp, you should have bought a decent pair of IEM's. basically what you have done is bought a half decent supercharger for a crap car with a crap engine.
the reason why people buy these amps is to use with a line out, not a headphone out.
a headphone out socket is already amped, you will be amping the same signal twice, but it goes through your devices amp first, therefore you will be just amplifying the devices amp, therefore no benefit of the E5's SQ will be used. so if the headphone socket is already amped why buy a portable amp? well the built in amp is usually too weak or made using cheap parts, so you bypass that using line out then amp it using an amp made with high quality parts.
i'll give you an example of how a portable fiio amp is used:
ipod - line out cable (plugged into the dock not headphone socket) - fiio amp - headphones
also the most important part of the setup is the source and the headphones, if one of those pieces is crap, then you can spend a million pound on an amp, it will make zero difference.
the biggest benefit you will see is from a decent pair of IEM's, then a decent source. also 99% of IEM's dont need an amp unless they are very high end or your using a DAC. so you dont need an amp unless you buy a half decent set of headphones at the minimum.
if you see earlier on in the thread someone posted some M50's, they dont necessarily need a amp, but will benefit from its use.
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