Where are the passive subs?

Anthony Gallo offers passive sub woofers intended to be used with their line of separate reference amplifier/crossover/EQ. I would recommend those. They are also bridgeable which is nice for versatility if you choose to go with a single sub.

Another manufacturer that offered separate sub amps and passive sub woofers is NHT. Not up to par with the Anthony Gallo offerings, but decent you can have some fun with them.

Dayton has both component sub amps built into a case that can reside on your rack with your other separates. They also have a few different plate amplifiers, with and without an EQ.

If your going to employ a plate with or without EQ personally I would spend more and get one from Hypex, one of the leading manufacturers of class D systems.

In the past I employed a 2ch A/B Bryston 4B with mini DSP and had pleasant results.

As mentioned some of the ready made to order subs kits that are out there can be a good way to go, with and without electronics. There are some well made sturdily braced cabinets offered with and without sub woofers or plate. Do it yourself sound group is one place I would recommend getting them. You basically just have to assemble your box they take some of the guess work out. But you will have to find a suitable sub woofer driver to match with the pre cut cabinet perimeters for optimum results. So some homework is involved there. Or if you feel more adventurous some sheets of your choice material and a table saw can be rewarding.

The Audiokinesis room with the Swarm Subwoofer system at RMAF 2015 sounded fabulous. I heard many high end speaker systems during the weekend and the Audiokinesis room was in my top 5.  If you normalized for cost they were, IMO, the number one or two best speaker system at the show.

Fernando