The Izophones-30S version is an assembly variation of the standard Izo-30 model. The difference between these models is exclusively in the degree of tension of the membrane. The membrane is stretched weaker in the "S" version to achieve a softer and more bass sound. In short, this is an attempt to try to adjust the headphones "to taste."
The sound differences from the standard model is in much more saturated low frequencies. Their level has risen. The feeling of depth of the bass was increased, but the whip was somewhat lost.
Needless to say, high frequencies couldn’t remain unchanged. They became softer, and maybe less smooth. Nevertheless, their level still remained at least not worse than the best representatives of foreign firms. Simply it doesn’t slightly reach the unique capabilities of the typical IzoPhones-30.