In my experience the sound quality changes drastically with an amp. I have tried it on iPhone 5S (FLAC/Onkyo HD player) but the cans opened up more in my transportable setup of iMod (Rockbox + FLAC) + Just Audio AHA120. I was quite happy with the transportable rig but then I plugged my iMod to NuForce HAP-100 and H6 shined. Got to try this with my Hugo but I am quite certain that this headphones deserve some quality amplifier to shine through, in my listening tests. That being said, iPhone 5S was able to dive these beautifully. I can easily understand if you chose not to amp them.
I also think that we tend to appreciate music in a certain way and we tend to gravitate towards equipment which shows us music the way we like to listen. I love analytical sound and almost always gravitate towards equipment which tents to have deeper bass but not as much slam and extended but sharper treble with good separation and soundstage. It is the reason why my favorite headphones have always been AKG-701, Etymotic ER4, Sennheriser HD-800, HiFiMan LE-5 and now B&O H6. Reason I am saying this is because may be someone likes their music presented differently. Trust your ears. Try them with various amps in a meet-up and then decide.