If you are married to that amp, see if it can operate in Tri-mode. Have not seen this application in many years(all x-over's are active nowadays). If you can replace the amp, go for a 4 channel and bridge channel 3 & 4 for the sub. This is the method I used with the MTX 704x I just installed.
Edit: Looks like the KAC 729s is Tri-Way capable. Now you need a Tri-way Crossover. Might be a tough find.