1) Connect the both speaker '-' binding posts to the PCB ground at the GND eyelet. You can connect both '-' posts together and then run one wire to GND. Note: The Darlings are usually floating and will benefit. A Chubby Clementine with no feedback will need it, but the ones built with feedback (like the ULtra) should already have this connection.
2) Run a wire from the GND eyelet to a chassis screw, such as a transformer bolt. Secure it with a star washer or a crimp eyelet connector.
These mods will drop the noise floor into the dirt. I'll update the connection documents.
Shannon