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

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

You could use a miniDSP 2x4HD, or even a DDRC-24 if your system could benefit from DIRAC. Enjoy!

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. 

Not sure why you would use a Y connector in your set-up?

At a minimum an electronic crossover will have 2 inputs and 4 outputs. Every powered sub that I have seen have both a left and a right input. You would connect your preamp outputs to the crossover inputs, then 2 of the crossover outputs to the sub inputs and the other 2 crossovers outputs to your main amps.

This will give you a little more overall output SPL, but the 80Hz crossover point may still be too low to get the SPL you want.