I own an iPhone 4 and also the SE535 Ltd Ed. With your 4S, I don't recommend an amp. I'm guessing you won't be able to get the volume of your iPhone much over 1/3 without it getting too loud (unless you have one of the volume limited EU ones).
The SE535 is both easy to drive and very, very sensitive. You shouldn't be getting any hissing because of the low impedence from the 4S headphone out. I have tried my SE535 with the $300 Arrow HE 4G and noticed no improvement. If anything, because they were so sensitive I had to lower the volume pot so much I risked channel imbalance.
If you want to hold onto your SE535, grab the Equaliser app (which has a very good parametric EQ - much better than Apple's limited one), and lower the mid-range/highs until you get the signature you're after. Otherwise I'd suggest selling them, and getting something closer to the sound signature you're after.
FTR - the standard 535's were definitely not sibilant to me (if anything their highs were a little rolled off which is why I went for the SE535 Ltd Ed). They do have a very forward mid-range though. I'd either look at your recordings, the EQ you are currently applying, or the tips you are using (olives should take a little glare away). If it's none of those - consider buying something a little more to your desired signature.