Adding a sub to modest system

I have a fairly modest system and I'm happy with it for now.  Most of my listening is background.  I have a Yamaha R-N803 receiver, Yamaha CD player, and Vandersteen 1C speakers. I even still have my old Sansui  SR 525 TT  I bought while in the service in the 70s. I stream Spotify.  I've tweaked my system a little with better speaker wire, better cables, a Furman power conditioner and better positioning of my speakers as per Jim Smith's book.  I'm considering adding a small reasonably priced subwoofer.  Any advice would be appreciated.  I really like this forum and although it is above my skill level, I check it every day and have learned a lot.  Stay safe and stay well guys, this damn virus will pass.