Let me preface this by saying that i use these headphones with a Chord Mojo which in itself has a very lean detailed and laid back sound. A more "boomier" DAC/Amp like the OPPO HA-2 would shift my findings. For example the White does not sound all too great with it. Everything is say here is of course IMHO!
Treble
Most -> -> -> -> -> -> -> Least
SR325e -> RS2e -> PS500e -> White
The SR325e has that kind of treble which tickles you ear. Tiniest details and fine dynamics get to be heard. With certain amps and sound materials this can become annoying. With my Chord Mojo it is however very enjoyable. The RS2e is pretty much the same but one notch subdued. Details are still there but less "in your face". PS500e and White are another notch down to a level which non-Grado fans might even consider "normal". It's no surprise, none of these Grados lack treble. They are all great treble-wise to my ears.
Mids
Least -> -> -> -> -> -> -> Most
SR325e -> RS2e -> PS500e -> White
The White is the master of mids. It makes me hear new details in songs i thought i knew well. The PS500e is similar but one or two notches behind. Compared to those two the SR325e nearly has no mids. The RS2e sits between the latter two. My favourites for mids are the White and the PS500e. Especialy the White is great for observing voices.
Bass
Least -> -> -> -> -> -> -> Most
SR325e -> RS2e -> PS500e -> White
Most and least also goes hand in hand with "detailed", the more bass the less detailed. The SR325e being the extreme with its very detailed "bodyless" bass and the White being the other extreme with lots of bass body but not much detail. The PS500e is my favourite, it has a very punchy bass full of body, still has some details and even some sub bass. RS2e and PS500e are my favourite in this department, depending on sound material.
Soundstage and overall feeling
If you like a big soundstage then the White is for you. The sound also feels less "direct", probably because the drivers are further away from your ears. The White puts you inside a cozy tube-y world where nothing is annoying. It's great for relaxed listening but not so detail-orientated in the highs and lows and focusses on the mids. But it doesn't matter, overall it's a great package for fun listening.
The soundstage of the other three is pretty much the same, it's still a reasonable amount of separation but not as much as with the White. The sound seems more direct, you are closer to what's happening. The SR325e is a super finicky magnifying glass for sounds, often you get more details than you need. But it is enjoyable in its own rights. The RS2e is a reasonable and nicer version of the SR series and so is the PS500e which adds more mids and a punchy deep bass to the mix.
So which one is the best?
Listening to only one of these headphones for some days makes me get used to it and not miss anything. They all have that Grado sound and which one you choose in the end depends on your personal tastes, your DAC/Amp and the sound material you like.
Do i need more than one Grado headphone?
I don't think so. But i think you want more than one
If i could only have one of my Grados i'd choose the PS500e. Or the RS2e, depending on my mood.
If i had to give one of them away it would probably be the SR325e. But then i would lose the ear-tickling highs which only the SR line has.
So do i feel like i have too many Grados? No, not really. I want to keep them all. But i am pretty sure i won't add any more. Even though that GH2 does look very nice... Fortunately i am immune to the wiles of the Hemp as i can't stand those flat pads on my ears
Thanks for reading! Feel free to ask questions. I like to geek out over things i like.