I've owned two 325is, two 225i and currently have a 225e. There's very little to choose between the sound of the 325i & 225i - they are both bright headphones. The difference in build quality (and price!) far outweighs the difference in sound. I personally prefer the 225i as it's much lighter on the head, although the leather headband on the 325i is much better than the plastic one on the 225i (this can be upgraded though.)
The 225e has more bass and is a little less bright than the 225i. I've not heard a 325e, but I'd be shocked if it sounded very different from the 225e.
Ouch! I was looking longingly at the 325e, and I own the 225i with e-drivers. I was thinking the aluminium might pull accuracy a step closer.
There is a wider frequency response on the 325e. While that makes no odds to what I will hear in range, I wondered why. It suggests the 325e could have a different driver. Unless it's the 325's improved acoustic chamber that allows it to operate with increased frequency range.
What worries me about the 225 range is the original 225 only ever gained a 4/5 star award. (Anywhere that I saw, that is.) The original SR225 received 4/5 from What Hi-Fi. The 225i gets 4/5 stars and a recommended from Hi-Fi Choice, (it's on the box of mine).
It makes me question the construct. The upgrades the 225 has over the 125 are driver and grill, (ignoring the pads). What I mean is this. I sometimes wonder if the 225 doesn't quite have the acoustic properties in its body to contain and fully harness the upgrades. Leaving it a little un-refined. Please don't jump on me to defend your 225e over this, and let me elaborate on my 225e impressions. I love the sr225i with e-drivers!
The thing is that our impressions change over time. When we buy new phones or any audio upgrade we are firstly struck by how much better they are. Then when accustomed we love them for what they are and are still hearing things new in the mix. Then finally we begin to see limitations in them.
E.g. I used to listen to my PC on ear-buds. I put SR225i/e on my PC and instantly I was blown away. I was not only impressed by the astronomical difference between only low quality buds and the Grado. I was astonished that the motherboard was pumping out such quality audio. Others are experiencing this too recently. Check out this thread for more on that.
http://www.head-fi.org/t/754695/asrock-z97-e-itx-ac-outperforms-my-bifrost-and-crack-hd-600
However I could hear where the on-board audio was letting go. Anyway, because of this I decided time to get a DAC, and bought the Meridian Explorer. Now I have had that for a while I am seeing where the ME and the 225i are falling short. I know I will never get the best from my phones without a dedicated headphone amp. However I am sure I can get more from the DAC with better phones. The ME is good DAC with its own headphone amp so 325e or RS2e should make is shine even more.
I just can't get past the fact that the 225 range never gained higher than 4/5 stars in pro-reviews. I have a sneaky suspicion the 225e might though, or it would be close.
RE the headband though. I think I prefer the 225 headband, over the 325 headband. I like the 225 headband on mine. Plus it will never have the leather scent; not that I object to leather in any way. I find the leather headband unnecessarily opulent.