12db/oct 80hz hipass filter

My subs are xover at 80hz 24db/oct by my processor.The processor also does the mains at 80hz, but at 12db/oct (this all stems from Lucas THX )

Could I build a 12db/oct hipass line level filter @80hz to go along with the 12db/oct that is in the processor for a match to the subwoofer's 24db/oct lopass ? the filter would go inbetween the processor and the main amp.

Is it desirable to have these two filters match acoustically ?
You want the acoustic response of your sub-woofer and main speakers to sum flat.

If your main speakers have an F3 at 40Hz or below you'll get closer to that if you add a 2nd order line-level highpass with poles at 80Hz, Q=.707.

If they're sealed stand mounted monitors you're probably close already.