- First the 325 were made in different variations including the black anodized aluminium and black wire mesh and the plain aluminium version. Both looked quite cylindrical even if they had a small reinforced ridge at the end, somewhat similar to the current cup design, just not as wide. These look a bit funky IMO.
- Then Grado released the Golden/Champagne colored SR325I version that introduced the current design with wider a ridge/reinforcement and black wire mesh. As already mentioned these were released for the 50th anniversary and looked really classy. John is quoted saying that he felt the 325-series was the weakest link in the range, and wanting to improve that particular model. This cup design later found it's way into the rest of the lower Grado-range featuring ABS cups.
- The 325IS was a a differently tuned/improved model strangely enough belonging together with the rest of the Prestige I-series (SR60I - SR325IS), seeing as the SR325I belonged to the previous non-I generation. First they released an unfinished alu model (which I am in possession of). These were infamous for oxidizing slightly, so they introduced a "test-run" or rather a limited run of the chrome finished cups, quickly followed by the powder coated alu cups which have a slightly sparkly silver/alu color. I can't remember the exact release dates from the top of my head.
- And just recently Grado released the E-series with completely new drivers, materials and tuning etc. I thoroughly enjoyed hearing that they finally introduced the use of polymer materials instead of the crappy ABS. So far these has only been released with the powder coated alu cups.
As far as I know all models have differently tuned/designed drivers. I personally love the sound of the IS models, but the I-series are also revered for it's sound. They are said to be way more harsh/shrill sounding than the IS-series. I haven't had the chance to A/B any of the different models and have only had quality time with my 325IS.
All models not specified differently, came with stainless wire mesh.
EDIT. I personally think IS stands for Improved Second Edition rather than Improved Silver Edition. It would just sound silly to name a model after it's color, but who knows