I think I got it right...let's try this...
Many subs have low level input jacks (line in) and low level output jacks (line out). Most of the line out jacks are straight pass-though of the line in signal, however some subs (such as SVS PB13-Ultra) have HPF on these line out jacks.
So, if the sub in question has HPF on the line out jacks, then I suggest the OP connect preamp>sub>power amp so the power amp receives a HPF'ed signal. That way, the crossover in the sub fully manages the x-over between sub and HPF'ed mains, and the mains amplifier has to amplify only a HPF signal rather than full range signal - biamp, if you will.
Just because there is a knob doesn't mean you should turn it.