Having mono blocks has nothing to do with the number of subs needed. It's more room and listening experience dependent. 2 subs are usually better than one, however they are considerably harder to integrate than just 1 sub. Once dialed in though, they can be fantastic.
Depending on how loud you like your music, the 2 REL Classics should be fine. They do look nice and would look much better than many other offerings on the market with your KHorns. I'd take the 2 Classics over a single 15" carbon.