Answering another question here reminded me of an old audio device I had, a dbx subharmonic synthesizer
The dbx analyzed the music being played and if it saw a lack of lower frequency sound it would create and add subharmonics to it. Back in vinyl days it was common to reduce the low frequency content to better fit the remaining content onto the record. Not so common a reason today, but for some reason many tracks don't seem to have the low frequencies you hear in a live performance.
https://dbxpro.com/en-US/products/120a
Might need a license from dbx but adding that to any Sub enabled room would be a good selling point, both nicer to listen to and more justification for buying the Sub or miniSub.
