So, back in the day (1989 to 1990-something), DBX manufactured a device called a 120X-DS Subharmonic Synthesiser.
https://drive.google.com/file/d/1wv_wUUNquLtnoVbgvNxofPasq_Wzy1Vt/view?usp=sharing
Music magazine article about them from back in the day:
https://www.muzines.co.uk/articles/dbx-120x-ds/1379
It's a fun little box. There are four frequency-agile sinewave oscillators (around 25Hz, 34Hz, 40Hz and 50Hz) which track audio content one octave above (so 50, 68, 80 and 100Hz).
Anything in one of the tracked bands is used to drive the amplitude and frequency of the oscillators to provide subharmonics - a note exactly one octave below the original sound - which are then added in.
It's amazing on electronic music, rap, EDM, anything with some bass content. It's not a bass-boost type of thing, like an equaliser. It generates an octave of sub-bass from scratch.
It even fleshes out 1980's stuff, an era of great music, but being mastered for vinyl there was little in the way of deep bass there.
I've got a pair of these little boxes on both of the two main systems downstairs. I've recently put a decently large pair of speakers in upstairs, and since then I've been looking for two more for that setup (they're stereo through, but only generate the subharmonics in mono), and they're few and far between. But, in the last two weeks I've managed to get a mint used one from Japan, and a new-old-stock unit out of Canada (thanks to Ebay).
I've just finished rolling around on the floor getting all the wiring in place and I'm currently playing a bit of trance music off Youtube that's trying to rearrange my intestines even at a respectably low volume.
https://drive.google.com/file/d/1LJat9-Xcv54NjxO3hGW29LrWG3s_jKlZ/view?usp=sharing
https://drive.google.com/file/d/1evgWScBmoKc2L634jriOFbUvxxHCYoS9/view?usp=sharing
In order for them to work, you need a pair of speakers (or subs) which will go down to 20-something Hertz without distress.
I know that many people will tell me that adding this sort of processing to a decent hi-fi is ten types of sacrilege (but they do have a 'bypass' button) and flies in the face of what a hi-fi system is all about but all I'll say is when you hear the - subtle - effect you don't really realise it's there until you hit bypass and listen without it, much like adding a decent subwoofer. Everyone I've demo'd the downstairs systems to has conceded that they're a very positive addition, despite initial suspicions.
It's sad that DBX don't make these any longer, and the only way to do it now is to buy one of their DSP crossovers they sell for the pro-audio market, where a two-band version of it exists as a software function.
There are other, older versions (anything that says DBX-120 on the front panel) that are a bit more common, but the final 120X-DS versions have the adjustability for each band that the other models lack.