So many variables as with all audio questions. I have enjoyed many single subs, but I was not happy for 2 channel until I got a Rythmik sub. Best sub I have heard in all regards. Wish I had believed what I read about them years ago. I’d love to hear REL some day, but I also want below 20 Hz extension with the musicality at my pricepoint.
IF Rythmik is properly dialed in, no boom in the room. Even when not, it still sounds really good, but calls attention to itself because gain or crossover is too high. Saying 1 sub is always bad, I call BS. If you have a difficult room with a lot of nodes, don’t know how to set up a sub, or didn’t get one that will sound good to you, then maybe you can only figure it out with 2 subs. Caveat though, the only sub I have bought 2 of is the one that sounds amazing as a single sub.
I read somewhere to buy the best sub you can afford and as many as you can afford, and I may believe that. For 2-channel, maybe 2 is the ideal rule of thumb with reasonable budget constraints. I finally hooked up my second Rythmik, but I don’t really need it in my current room. For home theatre seems like 4 subs may not be overkill to even out response at all listening positions.
The Rythmiks give you some control with parametric EQ and other settings. I’m blown away by their depth, slam, speed, and musicality, but most of all, ability to blend in and disappear when adjusted properly. I intend to integrate miniDSP at some point, but I have been happy with results adjusting sub amp settings using test tones and SPL meter. I would lean towards the best sealed SVS sub(s) I could afford with phone app EQ, if that convenience was important to me.