Add Active Link
The FET/RACK comes with the traditional RCA stereo link connector used in the classic units. Used in combination with the RCA battery powered Stereo Adaptor it's a simple way to link two units.
Adding the active link allows you to activate linking with a rear panel mounted push switch. It also will stereo link units without the need to super match FETs or purchase a third part linking box. More info can be found on the Active Link sales page.