I understand; "...RCA cables from the back of the stereo unit to the RCA cables on the Bose system in the back bedroom..."
Bear with me further, but to where would the serial cable be connected to please?
I am trying to establish how the Bose Built-INvisible System 172075 is getting audio signals and turn on signals from the LS20's music center.
I am tempted to suspect that the reason ZONE 2 is OFF is because there is nothing connected to the SPEAKER ZONE 2 jack as may be seen from the second picture provided above.
Additionally, it is likely that the jewel cube speakers are not connected to the LS20's Acoustimass module.
And instead, the Acoustimass module's LEFT & RIGHT OUTPUT TO CUBE SPEAKERS are feeding the 172075's LEFT & RIGHT AUDIO INPUT.
Further, the 172075 would need a "REMOTE TURN ON" signal on its pin 4(?).
Absence of this remote turn on signal would not enable the 172075 to work hence no sound from the speakers connected to it.
Please confirm above.