This is a great update to the original IEM. Etymotic mean what they say when they describe their SR as a reference class, it very closely adheres to the measurement driven sound profile of the ER4S.
Etymotic are primarily a medical equipment company, making high end measurement, hearing aid and protection equipment. They did pioneering research and measurements in the early 80s into the in ear canal hearing response. As a side product of this, they were able to leverage this unique early insight to create what is arguably one of the most accurate heaphone response curves. They already had unparalleled expertise in Balance Armature design for their measurements, and their IEMs featured beautifully tuned single balanced armature drivers (no concerns regarding cross over distortion!). Similarly, they already had good tips for medical grade seals. . . the ER4's 40dB of isolation is the most of any IEM that I am aware of including all custom fits.
When Etymotic say this is a reference headphone, they mean it. You are buying their hearing model, and the ER4 are a mechanical vessel of delivery.
I am pleased to report this new SR model variant retains the exceptional sound performance and closely adheres to the same Etymotic target curve. As an engineer, I LOVE that Etymotic includes measurements of YOUR headphones with the every ER4, so you can compare your classics with both your eyes AND ears :)
-THE NEW MODEL-
Both these new ER4SR /XR as well as the economic ER3 series come with substantial mechanical improvements vs. previous generation. The sound remains blessedly unchanged, but these headphones have been made much more convenient and practical. The cord is replaceable, but it's also much more flexible. Combined with the smaller body size, it's very practical to now wrap the cord behind the ear as with most modern IEMs.
With my ER4S, I had to use the shirt clip to minimize cord self noise (sometimes called microphonics, incorrectly), but still I found heavy bone conduction made these unpleasant for use while extended walking. With the new models, I don't bother with the clip, and the snug new cord management minimizes these effects such that they are a pleasure to take on the go. I'm still not sure I'd recommend them for walking around the city (because of the isolation).
-THE BOTTOM LINE-
If you want accurate in ear headphones, and the prospect of a deep insert doesn't frighten you, just do yourself a favor and buy either these ER4 or the ER3 as your budget allows. The ER4 are made in America and are channel matched to the reference curve to a higher standard, but honestly the ER3 (made in china) are excellent as well.