Lemonhaze is giving good advise. The Rythmik subs are a great value. Danny at GR Reasearch has a video on the subs that is worth watching.
IMO subwoofers and subwoofer setup are the most frequently misunderstood and poorly executed areas of the audio realm. Two is better than one and four is a swarm. Save up and buy an Audiokinesis Swarm and you will have the best bass extension attainable, there’s nothing like it! And you won’t be spending money on acoustic treatments for the bass.