I have RT55i mains, and a CS400 center.
I was using Monitor 5jr+ for surrounds, but now I'm using Monitor 7Cs (which are a little big).
I'm getting another pair of 5jr+, which I really don't need. So, in my dream world, I'd sell the 5jr+ and get surrounds that match the front. I wouldn't be adding much money to whatever I'd get for the 5jr+, so please keep that in mind when making recommendations.
Aside: why am I getting a pair of 5jr+ that I don't need?? Three years ago, my sister called and wanted to buy a basic music/movie system. I found her a pair of used 5jr+ right here in the Flea Market, and she bought a brand new Sherwood 5.1 receiver and a Kenwood 5-disc DVD/CD changer. Now, 3 years later, NONE OF THEM have ever been out of the boxes. It got to be a joke that whenever I talked to her I'd ask "so, you hooked up your stereo yet?" So, now she's simplifying life, getting rid of stuff, and asked me if I wanted all her gear because all she does is move the boxes around. I'm emotionally attached to my 7Cs, and my wife bought her 5jr+ when we were dating, but THIS pair may have to go... it would be my 5th pair of Polks (including the little M2s we used in the kitchen).