I have two subs, but typically only keep one hooked up mainly due to WAF, and I also have very limited placement options. My room is 20x 24 x9, and opens into another large area with a vaulted ceiling, so there’s some cubic footage in play. I prefer both subs hooked up, but one sounds better in my situation than none. It’s a treat when I run both subs and pull the speakers out where they sound best. You can buy a decent sub from Dayton or Monoprice for < $200 if you want to experiment before sinking more money into it.
Even low cost subs like the Dayton have variable low pass crossover frequency options, separate gain, and two phase options that help give some options to dial in the bass to suit your needs, even without many placement options. I run mine from the amps into the high level inputs, keep the low pass crossover frequency very low to avoid directionality and resonance issues, and keep the gain very low so it just augments the bass from the mains. I don’t "feature" the sub. It's subtle... I barely hear it except for on heavier bass passages.