I've used several amps with my Grados (I have the GS-1000, the RS-1, the SR-325i, the SR-80, the SR-60 and the iGrados). Because the Grados have such low impedence, I first ran them straight off my iPod. My first dip in the pool, as far as headphone amps is concerned, were with slightly upgraded Cmoys. I found a fair amount of difference in sound, depending on which opamp was used. This got me into opamp rolling, which turned out to be a lot of fun. Brown Burr has some very mellow opamps but I'm partial Analog Devices, whose opamps seem to err on the side of detail. Audiosnobs poop all over Cmoys but there's at least one article out there showing that - for $20 to $50 - you can get as good (or better) sound out of a Cmoy as out of Grado's own RA-1 headphone amp (sans the block of mahogany and the cheap, cheap, cheap opamp). To my ears, which are closer to champagne than Budweiser, the Cmoys do surprisingly well for the money. You don't get earthshattering bass (or world-class soundstage) but they do sound better than plugging directly into your portable device.
My next amps were Pimetas, which had more soundstage than the Cmoys, and a little better bass, though nothing overwhelming. The nice thing about pimetas is that they handle opamp rolling really well. One of my pimetas was standard and portable; the other was a pimeta "maxed" that was more of a desktop item.
My next amp was an M^3. This amp, which only cost me $300, had a separate control for bass boost which allowed me to adjust the warmth. Turned halfway up, the bass was about as hard hitting as anybody should ever want. Turned up all the way, it was like living next to Godzilla. It was with this amp that an old truth came home: There are tradeoffs. It's possible to have too much bass. At some point, added bass affects the mix, making the sound too warm, enough to make me feel claustrophobic. The nice thing about having a separate control for bass is the ability to adjust for different recordings. Some sound anemic without a little adjustment; others are fine the way they are, and turning down the bass helps maximize clarity.
The M^3 really brought out the best in my Grados, including my GS-1000s. There's an issue, partly based on preference and perception, that the GS-1000 is a tad sibilant. With the other amps, that issue remained in play, but with the M^3, my GS-1000s had unprecedented balance. For the first time, I was able to enjoy those cans for what they are. Like the pimeta, the M^3 can be tweaked through opamp rolling, something I've considered doing but have yet to feel motivated enough to try.
Right now, in the "for sale" section, there's a M^3 that has something mine doesn't: a finely regulated power supply. My M^3 has a basic elpac wallwart. The M^3 listed has a sigma 11. I've not heard the difference but from all I've read, it's a substantial improvement in sound.
My next amp will be a three-board B22.
