Thank you all for your feedback. In the end, I decided to go the safe way and got another pair of R-51Ms for surrounds, because knowing myself, if I got RPs I'll try to listen to the timbre mismatch and I'll notice it (or think I'm noticing it) and start to plan for an upgrade right away (I kinda started checking even new receivers). And that's a month after I just bought my new system I'm supposed to enjoy for a while. Coming from a JBL cinema 350 soundbar, this is already a big improvement with the 3.1 already, and wouldn't want to go down the rabbit hole of chasing the best possible sound (just yet).
I've read on a lot of forums how one should avoid reference series because they are low end, but to my ears, this is hi-fi really, so I'm a happy reference owner. And I really think the reference line sounds amazing considering the cost, not to mention the cool looks. Initially, I though of going with some cheaper Yamaha speakers, and a considerably cheaper Yamaha subwoofer but I'm glad I went with Klipsch all the way. And staying with the same line, was probably the safest way to enjoy the system without regrets.