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

 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.

@dcaudio ...Simple, this: more than you'll ever need even later
I've the Pro version, but the LE is cheaper, less if you opt for open box or used.

Source noted offers free shipping on some items, turn-around is quick, they are helpful, the warranty gets extended....yes, the learning curve is steep as noted in one review....
But in the process you'll get an education in the subtle art of a crossover.
The D series eq's are nifty, a future upgrade when I can get a 'steal this' $ drain. ;)

Enjoy whichever you choose, J