Sine DX sounds better with an amp. iPhone or even Sony ZX2 doesn't quite do it. Don't laugh, the ZX2 actually does well into lower Z loads - it just don't like high Z phones because it can't crank up the voltage.

From a portable, the Sine DX sounds slow, plodding, and compressed. Less so even when given a decent amp (Schiit Fulla for example). Still, the Sine DX does still have this tendency to a be a bit slower and compressed. It is also a little bit less resolving than other dynamics I have on hand: Sony MA900 and HD650K.

It still sounds closed. Not claustrophobic by any means, but essentially sounds closer to a closed headphone than open headphone.

Think Audeze LCD series, but a lower fidelity, but super portable version. I wish transient response / speed was better on the Sine DX.