Hello Gonk - The SA amp is driving the main left/right speakers, the Nucleus Reference 3.1's. The following is from the manual (to help explain)

http://www.roundsound.com/manuals/reference3-manual.pdf

--- "The lower binding posts are labeled "Sub In" -- they connect to the Nucleus Reference3 woofer's secondary voice coil - they're designed to receive power from the optional Anthony Gallo Acoustics' Reference3 SA power Amplifier. The SA amp works independently from your main amp and dramatically enhances the Nucleus Reference3's deep bass performance -- down to the range we associate with full-size subwoofers. The SA amp will allow the Nucleus Reference3's to reach 22 Hertz."

Thanks for the help with the second trigger and I will use the TR2 with the preamp SW1 output on the 990.