who currently makes / sells swarm style subwoofer systems?

The AK system is a passive system with a centralized amp, but I imagine you could do the same with just about any set of 4 active subs, instead.

@audiocanada is completely right. In fact, some people prefer to have four separate amplifiers to tweak the phase of each subwoofer based on location. So you would be fine with four independent subwoofers.

James Romeyn Music and Audio offers systems based on the asymmetrically distributed multi-sub concept.  

AudioKinesis has, at times, worked with customers to find semi-custom solutions if the "off the shelf" one isn't a good fit.

And of course the basic concept can be implemented in multiple ways, as has been mentioned. 

Duke

Your room's modal spread dictates that every sub has a specific location. There is no willy-nilly 'random' asymmetric placement.

just go with active subs...

Here are some cheap subs that can't get beat for the price...should take xlr cables (for long cable lengths) and have variable phase control, very important.

https://starkesound.com/products/sw12

https://starkesound.com/products/sw15

P.S. you could run multiple passive subs with a single multichannel amp...but, now , you will need a processor, etc...There is also a chance you may end up with low quality subs.

Audiokinesis Swarm is an excellent choice for swarm subs - well established with plenty of positive testimonials, deep knowledge, great support, reasonable price in high-end audio.