I've had the RS1i for coming up to two weeks now. I was so impressed with my SR225i I brought the RS1i after reading many posts which suggest the RS1i is a good upgrade over the 225's.
I haven't done any AB'ing but I think the improvement over the 225's is fairly subtle. They're a touch more neutral and rounded - and consequently less bright, with greater bass impact and a small increase in detail and airiness. I think they build on the best bits of the SR225i while moving somewhat in the direction of HD 650, another favourite headphone of mine (though I haven't used them since getting a pair of Grado's). With the RS1i I can listen all evening long while still enjoying the amazing way Grado's render guitar and communicate the emotion in the singer's voice.
I haven't found a genre I don't like with the RS1i, it does a good job with blues, classic and progressive rock, jazz and pop. Some recordings can still sound a little anaemic when compared to the HD 650 - but that's only because they are anaemic recordings. The extra body provided by the RS1i over the 225 does provide an improvement here.
I would say you're more than two thirds of the way there with the SR225i though, which makes the 225's great value in my opinion (and conversely makes the RS1i not such a great bargain, if I'm honest - but we're all familiar with the laws of diminishing returns here!).
Thanks to the thicker leather headband I find the RS1i is a good deal more comfortable than the 225's too. The only real discomfort I suffered with the 225's was the thin metal band pressing against my head through the thin vinyl cover.
I must confess a not insignificant draw to these headphones was the way they look. The mahogany cups really work well with the unique Grado styling, giving them a classy, vintage appearance. After all the comments I've read about Grado build quality I was a little concerned whether they would have any defects, but I'm pleased to say these look like a perfect example to my eyes.
I've been using my AMB M³ to drive my RS1i (and SR225i before them) and I'm very pleased with the pairing. I also have the Little Dot I+, which I know is a very popular amp to pair with Grado's. There's no doubt the I+ offers great value for money, but I find the bass lacks definition and it's not a very detailed amp by comparison. Personally, I think if you're going to shell out for the RS1i, it's also worth shelling out for a better amp to get the best out of these headphones. I haven't heard many solid state amps, but I think the M³ has a signature not dissimilar to tube based amps - it's certainly not overly bright. I've read MOSFET output stages have similar characteristics to tube output stages, so perhaps that's why.
Edited by jr41 - 6/21/11 at 1:23pm