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.
Rev D Orange Drop Mod
The FET/RACK revision D includes capactiors selected by ear to match the sound of the classic units. I some cases, builders may want to add even more color and agressive tone to the unit to match their tastes. Replacing the four stock coupling capacitors with Orange Drops is a simple and easy mod.