I have no idea. There are so many brands and models of everything, I stopped paying attention 20 years ago. Seriously. The only things I can discuss knowledgeably are those I (or my friends) own. That's probably less than 1 percent of the total number of audio products out there!
My issue with a subwoofer for 6" monitors is now we've entered the 8" price point. Also, now I have yet another piece of equipment to deal with. But maybe there's a good reason sonicly to go this way?
I have a pair of 8" Yamahas that say they're +/-1db at 50Hz. I just added a pair of Yamaha subs that are supposed to be down 3db at 30Hz. I've got them dialed in pretty well (still looking for a measurement mic) with some crossover action to take a bit of load off the monitors and there is a noticeable extension to the very low room shaking range. I'm lucky to have a room that works pretty well even untreated, and am pretty happy as is. It's gonna really cool with a bit more treatment.
KRK says the Rockit 6s go down to 35Hz, but don't specify what that means. If they're actually flattish that low then one shouldn't need a sub.
One thing to say, I suppose, is that a suwoofer acts as a kind of an upgrade path. If you've got smallish speakers with which you're familiar and otherwise satisfied, a good sub can get you bass extension without replacing the whole set. Or if you cant afford the bigger speakers, you might start with the smaller ones and add a sub when you can afford it.