How big is the room? I might split the difference and go with 2 subs that aren’t necessarily Rythmik fv18’s but also not 4 crappy ones either. I actually have a 505 somewhere in my house. For the money, and I stress that, for the money, it’s OK. For the inexperienced, it seems to have a lot of bass, but once you experience linear clean smooth bass, it’s a fart box. When the Polk is pushed, it makes objectionable noises, and really isn’t great and actually more of a distraction. It also falls off a cliff at about 25hz.
If the room is small, the rsl speedwoofer is 400 bucks and 4 of them might be great. The Dayton sub1200 is also a decent budget model.
For the best value imo, HSU wins.
The best value for price home theater systems and subwoofers on the market guaranteed
www.hsuresearch.com
Slightly over your listed budget, but this pair would be better than 20 psw505’s, as they’d never be able to match the quality and control of bass even if all 20 could go louder.