I found a great REL sub at The Music Room. They also helped a friend of mine decide what size sub he needed. In other words, they're very helpful and have great deals on a variety of different brands (what they sell varies and they only take in good quality stuff). I would suggest you contact them and start a conversation. They have a 14 day trial period and all gear is checked out in advance. I do not work for them, I swear! I just find them almost unique in terms of great deals, variety of gear, helpful customer service, and a return policy.
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.