Electronic cross-over on pre-amp out, to split signal to subwoofer and bookshelf speakers

I built my own 80 Hz (second order) high pass filters from a few high-quality components.

DM me if you want the schematics and parts list. (My version is a bit complicated because I have a balanced connection between my preamp and my power amps but it can be simplified if you use single ended connections, essentially using only half the number of parts.)

My preamp has two outputs so I use one to go to the high pass filter and then on to the power amps (I happen to have two monoblocks) and the other output to the subwoofer (which obviously has a adjustable low pass filter already built in).

I guess you take some load off the power amp, but most importantly you are not forcing your main speakers to deal with frequencies below 80 Hz, where most main speaker drivers don’t shine.

I personally think this is a great way to "bi amp," by using the regular power amp (or amps) for the mid-bass and up, and using the built-in sub amplifier for the bass below 80 Hz (+/-).

Check out Sublime Acoustic. They make two different levels of crossover unit, the K231 and the K235. Lovingly crafted in Northern California.

The one issue that some people complain about is that to change the crossover point, you need a small card they provide, with precision trimmed resistors on it. A different card for each frequency. You get two with the purchase, and the rest are just $10 each. 

I have a set from 50 Hz to 80 Hz in 5 Hz increments. They are easy to change out, and crossover points are not something you want to A/B quickly, anyway; you need to listen a while with each choice in the system. I really don’t find the slow change-out to be an issue.

I find the units to be very reliable and transparent.

JL audio CR1. The best. Check out the reviews. 

While what you propose is technically possible, if I were you, I would not go forward.  It will probably not sound as good as before.  Using a 80Hz crossover point will still send a substantial bass signal an octave lower (40Hz) to the bookshelf loudspeaker.  It's not going to gain you much headroom.  Use a steeper crossover and you run the risk of creating phase issues.  Crossover at a higher frequency and you might degrade upper bass/lower midrange power region.  Every practical loudspeaker design has a set of built-in compromises.  I'm suggesting that those picked by Vimberg are a better set of compromises than you could improvise.

If you were just looking to add more deep bass to you system, then go with something like what REL offers that lets your existing loudspeaker run full range.

 I built my own. I ordered the circuit board from Elliott sound products and tracked down all the parts from different suppliers. I love it and would not go back