I owned the SE425 and currently have the SE535 Special Edition. I use an iPod Touch 4G and iPhone 4.
I have owned the Fiio E7, E11, and recently the Arrow 4G.
Just my opinion (others will no doubt differ) - but in direct comparison (trying to volume match by ear) using the E11, Arrow and straight out of the iDevices, there was no sonic advantages to amping. All it did was add bulk to a portable solution and a hardware EQ (bass boost) etc. IMO it does nothing for sound stage, dynamics etc.
The SE series are very easy to drive, and very sensitive. You may encounter hissing from devices with high output impedance - but I did not encounter any from either the Touch or Iphone.
IMO - as actual past owner of the devices you're talking about - forget the amp, celebrate the fact that you now have a truly portable solution, and spend under $10 on either the EQU app or Equaliser app instead. Both give you an excellent parametric EQ which you can then use to tweak the sound to what you prefer.
BTW - advice for future. For when you get suggestions from the forum (no matter how well meaning they may be) - check to see the poster's profiles, and see if they have experience with the products they are making suggestions about. I say this with particular reference to this:
As I'm sure the OP can attest to by trying the 425's straight from the headphone out of his 4S ..... there is no hiss due to the 4S low impedance, and with EQ removed from the equation and volume matched, I sincerely doubt in a dbt that one would sound "better' than the other.