So wo sub out, you are attempting to extract sub info from L+R, is that it?
U can use a Y-cable splitter and send line out to active sub assuming sub has built-in crossover, but L+R remain to receive full-range signal.
More ideally, sub does crossover for whole system, and passes-thru high-pass signal to L+R but needs amplification for L+R unless L+R are active speakers.
Must keep line level and high level signals separate.