I used to have the R30s and a CS2 center, with R15 surrounds, and for what I paid for the system, I thought it sounded really good, and I also found the R30s to have decent bass for what they were. Even though the R50s have an extra 6.5" woofer, I still doubt it will have enough bass to not use a sub, and I also doubt the Monitor 50s will as well. I would stick with the R50s for the rear, just to keep everything voice matched, and the sub 100 or sub 120 are good subs for the money.