Separate names with a comma.
Which has the loudest volume right out of the box and connected to Mp3 source?
The E4, because it has a much lower impedence.
what's impedence?? >_< according to headphone.org...
Impedance @ 1kHz: 29 Ohms
Impedance @ 1kHz: 27
basically... are there noticeable difference between the two in terms of volume?
Oh, I thought you meant ER4S. The ER4S has an impedence of 100ohms, and there is a noticable difference between it and the E4s.
ER4P and E4C/G should be similar in this matter. but really it shouldn't matter, because both can be well driven out of any source. The ER4S on the other hand, has 100 ohm impedance, thus requires amplification.