Using towers instead of bookshelves will sound better, they will have better bass extension. Plus if you can keep 6.5" woofers all the way around, that is even better too. You can always go to the Monitor line, but the R series will be a better tonal match to what you already have, but yes R15s or R150s as sides and R30s or R300s as rears or R50s or 500s as rears will be good too. If you stay in the R series, it will all sound good. I used to have R30s as fronts, and R15s as surround and surround backs, and it sounded great, and I was using a CS2 center since I couldn't get a csi25 at the time, but the CS2 matched well enough and still sounded great.