Bengkia369
1000+ Head-Fier
If I were to guess, might have something to do with output impedance. Not sure about the output on the Pioneer, but on the iPhone SE it's about 1 Ohm or less if I remember correctly. While I won't say the ER4SR are especially sensitive IEMs, this might be one factor making a difference.
And yes, like @Voyageur said, the iPhones and iPods for that matter are actually really good players, contrary to what many audiophiles may be led to believe. Those Cirrus audio chips are actually quite decent.
Pioneer uses Sabre ESS if I'm not Wrong but I really don't like the sound. Opus #2 uses similar Sabre DAC yet sounds very different.
It's not what decoder chips use that determine the sound, it's how the whole implementation.