Slave-Out of my old 2 Channel Rectifier was grabbed behind the poweramp, if i remember correctly. Pretty sure about that. So: Slave-Out = Poweramp-Signal.
"This 1/4" jack and control provide a signal derived from the speaker jack. Perfect for using
either the DUAL or TRIPLE RECTIFIER Solo Head as a master pre-amp, or additional power amps may be connected for
more power when needed. Some players use this to derive an FX Send Signal and go to other amps for their wet
sound." - the manual.
Regards