I've got a few S9j boxes running underclocked at ~700W. It's a standard setting on a number of the custom firmwares out there. (I use Hiveon because it's easy and mostly decent.)

https://hiveon.com/asic/#S9

That shows you some of the prebuilt setups they have.

As people have noted, they are not even REMOTELY as profitable as newer miners: at best, you're looking at 77.3 J/TH vs 22 J/TH (or whatever). That said, they work great as space heaters that pay you a little money.

I'm running a number of the boxes in these shrouds: https://www.cryptocloaks.com/the-future-of-space-heaters-s9-bitcoin-asic/

Next steps are to tear apart 19-series boxes and use Loki hacks to get profitable space heaters that run on 110V: https://pivotalpleb.com/collections/all

Good luck!