I have a HPF from Marchand Electronics for that purpose. It's a 24db slope at 80hz. It's completely transparent and accomplishes what you want. My preamp has 3 outputs so I have a separate one for my sub.
Electronic cross-over on pre-amp out, to split signal to subwoofer and bookshelf speakers
I have a 50 W amp for 86 dB bookshelf speakers.
I really want to keep both amp and speakers.
My idea is to have an electronic crossover on preamp out, split the signal say at 80 Hz - Send the lower electronic cross over out to a subwoofer. Send the high crossover out to my existing 50 W power amps.
This would extend speaker frequency range to subwoofer capability - say 25 Hz, reduce the power amp load and allow to play the bookshelf speakers a bit louder,/ have more headroom.
I am sure others have already done. What cross over did you use? Was it successful?