The Impedance Switch Kit is no longer available. While it was a great idea, we have many different versions of the SEX amp and two different versions of the Stereomour amp, and the complications of making a manual to work with four or five different kits got to be too much (especially considering how many of these we were selling). The small PCB toggle switches also frequently went out of stock. There isn't a whole lot of use for the 16 and 32 ohm settings, and we found ourselves explaining over and over again that 32 ohm Grado headphones absolutely should not be used with the 32 ohm tap. This is why you see the recommendation to use the amp on the 4 ohm setting if you aren't sure, or the 8 ohm setting for speakers.
That is the short version of why you don't see this kit available anymore. The Mainline circuit is a bit different and uses a different transformer. You could very likely wire up a rotary switch on the SEX amp to go from 4 to 8 ohms, but that's not the kind of project where we can give you three simple steps to get the job done!
