My understanding is they are essentially the same drivers, matched and selected better (and the housings are tuned better).
Grado has reported differently - they claim the drivers to be "different" (but they don't really elaborate) - my guess is that it's more like you're guessing - better matching and higher tolerances. That said, the SR-225 is the closest matching (just like the models above it), and my understanding is ripping up the plastic housings is easier than breaking the SR-325 or RS-2 down (and it's also quite a bit cheaper with the SR-225!).
To answeryour question about SQ as far as matched driver's, the 60's up to 225's driver's are matched .1db where as the 325's and up are matched .5db,
Actually the SR-60 through the SR-125 are claimed to be matched within .1 dB, and the SR-225 and up are claimed to be matched within .05 dB.
http://www.gradolabs.com/page_headphones.php?item=c56e04009f99e060f4c39dfdadb98549
In the Grado thread this question came up a while ago, and the SR-225 was the general consenous as the best starting-off point for transplants (because of the better driver matching, and as you note below, cabling and pads). I believe a few members regarded the 325 drivers as brighter even when liberated, so that's also worth considering. I haven't honestly read much about RS-2 or RS-1s being gutted for DIY woodies or similar - probably because they start out as woodies (and $500 just to rip a pair of headphones apart is pretty rich).
also the frequency responce is higher and lower from the 225's and up plus the wire conductor's change as you go up in model, also the voice coil wire's from the 125 and up are high purity long crystal oxygen free copper, and from the 325 up the connecting cord is also (UHLPC). From hearing the 80's to the 325's I definitly hear a big difference with the more closely matched driver's as far as stereo, where as with the 80's it seemed like I had more sound coming from the right than the left driver alot of time's and it wasn't the recording's (I am not a graph) this is what MY ear's heard. I don't know if you will hear a difference ( I don't how you hear), that being said I would either go for the 60's or if you want the copper voice coil's the 125's and you should be fine. The least you can spend on Grado's for the more closlely matched driver's is 300$U.S., for 325's, it may not be worth it just for the driver's price wise if you are going to take the driver's out of their aluminum cup's. Like another post say's I beileve you can buy Magnum driver's also if you want for around 120$U.S., minimum, I beileve they also have other higher priced model's too (I've never heard them). But like I said as far as I hear yes there is a big difference to my ear's. All of these model's have great SQ IMO.
They should all have copper voice coils, but I think you're right that either the 225 or 325 has an improved coil setup. On their web-site Grado lists the SR-125 as HPLC, and the 225 as UHPLC, 325 gets UHPLC cabling as well.
But I agree that they're all good candidates, especially if you're going to change the enclosures, pads, and perhaps even cabling (if you're doing a custom wire job, who cares what you're throwing away, is basically the point) - the SR-225 or SR-325 would both be good candidates because of the better driver matching, bowl pads (125 and lower come with comfies), and the 325 has the better cable and headband too (that should also be noted - the SR-225 and lower have vinyl headbands, and imho the leather headband does make a difference for overall fit and feel - but again, if you're going aftermarket here, who cares what you're pitching out).
I think if you aren't intending to go with an aftermarket cable, headband, etc - the SR-325 will have more parts that you'll appreciate on the rebuild. But if you're going fully custom, the 225s are probably going to save you $100 (to put towards the custom headband, cable, etc that you're into).
They do publish different frequency ranges for them... but honestly, having a set of SR80i and 225i, once you are using the same earpads on them, I'm hard pressed to tell the difference. I prefer my 225, but mostly because I've made ergonomic and usability improvements.
And see, this is the other side of the coin - and that's where I'm not as on-board with Grado's claim that they're all different drivers. Personally between the SR-60 and SR-225 with the same pads, I found the 225 to sound somewhat better, but it wasn't this big night and day difference. Sure, the GS-1000 and RS-1 sound different, but they also have much different housings (and in the case of the GSK, pads as well).
