Using a subharmonic synthesizer needs to be done with care and in appropriate circumstances. The Bassboss subs reproduce the music you put into them. The subs don't benefit from the enhancements, the music is what's enhanced, and it's the audience that can benefit. The subs just do their job... But there is a limit...
The key is, as you say, to only enhance SOME recordings. Often older releases can benefit from adding some of the deep low-end that's more common in recent releases to make them sound more current and less dated... BUT... adding subharmonic processing to tracks that already have deep bass, and lots of it, is asking for trouble.
Bassboss subs do very well at reproducing the low frequencies in even the most bass-heavy releases, and therefor do very well reproducing the enhanced low-frequency content of older recordings, but when you combine the subharmonic synthesis with music that's been produced with that kind of deep bass already, you start to ask your subs to reproduce frequencies that go well below the capabilities of almost all subwoofers.
If you run 60Hz through a subharmonic synthesizer, it will produce demand for 30Hz. 40Hz will demand 20Hz and 30Hz will create demand for output at 15Hz! The reason why older recordings don't contain as much LF content as current recordings do is because the technology to produce, record, and ultimately reproduce those notes didn't exist, at least not in any practical form.
The reason why current recordings contain a lot more bass information is because the technologies to record and play them back are now a reality thanks to digital recording and playback equipment. The loudspeakers to reproduce deep bass are also now available, although not yet common. Tracks that contain a lot of content at or below 20Hz are still very rare for several reasons, starting with because the frequencies can't be reproduced effectively for reference in most studios. Those notes also cause the majority of systems that would be used to reproduce them to distort, or fail, making the songs sound bad, so the producers filter them out.
If you then add them back in, you'll likely cause the distortions and failures that filtering them out was intended to prevent. Unless or until you have a loudspeaker system that's specified to reproduce frequencies below 20Hz, I'd recommend using any subharmonic synthesizer only to enhance bass-lacking recordings or live instruments, and be very careful how much you use. You might think of it like adding salt in a recipe. A dish without salt can be bland, so adding some will improve it, but adding a lot of salt to a salty dish will definitely ruin it.
Get it right and you'll be cooking with bass! Get it wrong and you'll cook the bass...