Not to complicate things, but Wayne is spot on. You will not increase extension by adding subs. Ain’t gonna happen.
Now, the complication: most of the Subs you see us recommend here are high quality Subs. This nonsense that only Rythmik is good for music, but not SVS or Hsu or Outlaw... well, it’s just nonsense.
I would recommend 2 subs now, that dig deep enough for your taste. Do not sacrifice extension for more quantity of a lesser sub. You can save up and add a third next year!
Hsu is absolutely the value leader in the Subwoofer market. I have never heard a complaint about their Subs. And the VTF3 is a great option.
The Outlaw and Monolith Subs are also highly recommended. I am uncertain about sales on them right now, but Outlaw will usually run a Christmas/New Years sale and has the X13 for $1000 ea, shipped. Monolith was running a BF sale and might still have them going: I would look at the 12” or 15” options if you can.
Regardless: Shop for the best Sub that you want. Buy two now if you can. Save for a second or third later.