How can I lower the sheen of my topcoat to a satin?

You can reduce the glossiness of a sheen several ways.

1. Apply a flatter sheen over a glossier sheen, such as Satin over Semi-gloss. Be sure to observe proper dry times. If you are applying over an existing finish, be sure to prep sand and clean first. 
2. If using General Finishes Arm-R-Seal, you can intermix Satin and Semi-Gloss to create a sheen that will softly mute the glossiness.
3. Another option is to rub the surface down with 0000 steel wool and orange oil or any similar lubricant such as Butcher Block Oil.