I second what MalVeauX has said.
You'll read a lot here on Head-Fi that Grados don't need an amp, and that's because it is true. I honestly have a hard time discerning an amped Prestige Series Grado (and RS1i for that matter) and one straight out of an iPod. If I noticed any improvement in sound, it was so miniscule that I would never have missed it had I not owned an amp. Save your money on the amp! Seriously!
I've heard every in-production Prestige Series Grado, in addition to the PS500, RS1i, and RS2i. I even started with a SR225i and upon looking to upgrade, got the SR325is, which I liked more personally. Then for fun I bought a SR60i to see if I could with confidence recommend it to friends, and to my amazement it sounded great! So great, that I actually sold my SR325is and SR225i and kept the SR60i for various reasons - sounded more "coherent" to my ears, better balanced, less V-shaped, tighter perceived bass (probably due to there being less of it), less sibilant, and overall smoother in the highs than the upper models which might actually be due to production variation.
Speaking of production variation, I want to mention that I think in a given Grado model, there will be variation in sound from headphone to headphone. That means one SR225i might sound a bit different from another SR225i. For instance, I recently heard a demo SR225i at the local dealer and it had way more bass than the SR225i I had before, to the point where it sounded quite warm. I've also heard two different pairs of RS1i on the same equipment and one sounded vastly more distant sounding than the other one, and the more intimate sounding one was much more fatiguing. Also, my particular pair SR325is did not have razor-sharp treble, as is often suggested to be the case here on Head-Fi. Mine actually had very good treble and I felt it was better controlled than the SR225i and SR80i. Actually, I do believe the statements when people say they heard razor-sharp treble with the SR325i. I just chalk it up to production variation, or to the fact that the newer SR325is is legitimately better in the highs.
For all these reasons, I suggest SR60i or SR80i with L-Cush pads. Having compared SR60i to RS1i directly, the SR60i does not pale in comparison like you might expect (and like I certainly expected). I was shocked to be able to honestly say the SR60i even reaches about 80% the quality of the RS1i. Thus was my impression in an A/B session I had at home. My 80% might be someone else's 50%, for what it's worth.