Why not just use a sub that has speaker level inputs?
3 responses Add your response
With separate preamp and amp it’s easy.
Preamp to L and R line ins on sub
Sub L and R line outs to power amp(s) L and R inputs
This way the sub rolls off the mains, typically with a 2nd or 3rd order high pass filter around 80 Hz, and the subs take over below 80 Hz, depending on where the sub’s crossover is set.