I just remembered one pre/pro which has separate sub-woofer outputs. Its feature-set is a little dated now, but the Theta Digital, Casablanca III can have up to 12 outputs, mapped from any number of input channels, with individually set crossover points. So you could take the left input and send >80 Hz to one output, and then the <80 Hz also of the left input to a separate out. It can have front/rear subs, or left/right very easily.

Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs
-Chris