IMO you're better off with an active sub. I think many active subs have high pass out, so you run pre out to the sub inputs, then run from the sub high pass out to your monoblocks.
I run a REL subwoofer which takes a slightly different approach to integration - you run your speakers full range, and set the subwoofer x-over point to come in below your speakers. You can also run the REL using high level inputs (ie tapped to your amplifier speaker outs), or low level from your pre-outs.
You will want to read the manual of any subwoofer you're considering buying to see how it connects and what they recommend.
I run a REL subwoofer which takes a slightly different approach to integration - you run your speakers full range, and set the subwoofer x-over point to come in below your speakers. You can also run the REL using high level inputs (ie tapped to your amplifier speaker outs), or low level from your pre-outs.
You will want to read the manual of any subwoofer you're considering buying to see how it connects and what they recommend.