I use 2 subs. I wanted something lively and quick, and that would fit into a very small spaces, so I got a pair of MIrage MM8s, which are 9" cubes. They don't go down to 20Hz, but they fill out the 32-50 Hz region where so many speakers are down 10dB or more. As with your experience, I found these $800 subs on sale for $270 each, so I also thought, "Why not buy two?" They have aluminum cone 8" drivers in sealed cabinets supplemented by two passive radiators. They're tight and quick and blend easily with the main speakers.
I position mine against the wall, each directly behind its corresponding main speaker. This gives me significantly more room reinforcement than placing them alongside the main speakers. These subs have continuously adjustable phase from 0-360, so dialing in the time alignment was painstaking but not really difficult. I get more low end drive or rock and pop and better scale and foundation for orchestral music. Listening to Beethoven's "Eroica" as I write this.
I position mine against the wall, each directly behind its corresponding main speaker. This gives me significantly more room reinforcement than placing them alongside the main speakers. These subs have continuously adjustable phase from 0-360, so dialing in the time alignment was painstaking but not really difficult. I get more low end drive or rock and pop and better scale and foundation for orchestral music. Listening to Beethoven's "Eroica" as I write this.