It will soon be 2019 and you're considering flip phone subs.
Subwoofers with room correction that are adjustable from your listening position have become affordable.
Every subwoofer suggested here can do the job, but how easily and how well they can be integrated to your system and your room is questionable. Compare their setup procedures then look at the procedures here.
http://syzygyacoustics.com
Or here.
http://velodyneacoustics.com/pdf/digitaldriveplus/DD+Manual.pdf
Subwoofers with room correction that are adjustable from your listening position have become affordable.
Every subwoofer suggested here can do the job, but how easily and how well they can be integrated to your system and your room is questionable. Compare their setup procedures then look at the procedures here.
http://syzygyacoustics.com
Or here.
http://velodyneacoustics.com/pdf/digitaldriveplus/DD+Manual.pdf