Very nice!

Winter my friend. Statics got you down.

To properly implement a sub you need to roll off those frequencies (approximately 80hz and below) from your mains. If you do this, you are saving your mains from having to do the large excursions needed for those frequencies and allows them to produce a more accurate midrange. One way to do this is to put a filter consisting of a capacitor between the pre amp and amplifier. The cap value is calculated based on the input impedance of the amplifier. You can do this with separates as well as any integrated or receiver with a pre out/main in.

The plans come with a crossover schematic if you go with the passive version.