Are you trying to say:
Iriver + se215 = flat sound?
Galaxy s2 + se215 = full bass?
I'm pretty sure the s2 has got a weaker amp than the iriver. Did you EQ your s2?
If you want to keep the iriver, you can get an amp like .22 / .25 or RSA Predator.
otherwise, would recommend you the x19hd player, fun signature with big deep bass.