Subwoofer recommendation for swarm

I want to create my own subwoofer swarm am looking for reasonably priced units, want to stay under 2K.  Recommendations welcome.  My room is 20' L X 13.5' W X 9' H. 
So really after digging around, it seems that the swarm method could be accomplished with 4 less expensive subs like the kit ones from Dayton Audio or built ones the Klipsch`s , Polks etc..

Providing they have speaker level inputs and are powered ? 
Why is speaker level inputs a requirement? Can I just split the LFE signal from the receiver into 4?
¨ was able to compare them wired for 4, 8 and 16 ohms. While they deliver more power into 4 the bass was much better by far at 16. According to Duke none of his customers who have compared have preferred 4 ohm. Something to keep in mind when comparing speakers.¨
 @audiokinesis @millercarbon
Hi just started into the swarm journey, I have already two Vandersteen 2wq subs in my front corners and i use 3 4ohm  B&W passive subs, one in the top of a bookshelf and the 2 on the back wall. A total of 5 subs with the dayton Sa1000. Now i am connecting them in series making the load 12 ohms. I,m confussed if it will it be better to get another 4 ohm sub (4 pasiive subs) and  connect them in series to get a 16 ohm load or in series parallel and have the load at 4 ohms. Right now i feel the front subs are stronger in character even at a low setting and I almost don, t hear the 3 on the back, volume on the Dayton is about half(12oclock).  What do you gus think?