I would agree, you have a challenging room though as its rather square.
1 very good subwoofer in the rear right crossed over below 80hz may work.
But there are swarm subwoofer options where you go for four 8 inch subwoofers spread throughout the wall space and you get a much more even bass response, but you are not going to get much below about 22hz.
Whats your budget ?
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