So i'm in the market for some new speakers and the reviews on the Polk Monitors are pretty good and they are decently priced. My question is I know a guy selling Polk Monitor 50s for $100 for the pair, has Monitor 60s for $200 a pair and sometimes has a sale for Monitor 70s $300 for pair, my question is, is there a substantial difference in the 3? I've heard monitor 50s lack in low end, but i have a nice sub to help out so thats not much of an issue. I'd be using this for all around use, home theater, music, games...etc. I have RM7 in the rear and a CS1 center channel. I also came across a guy selling Polk LS 90's for $300 for the pair, anyone know if this is a good deal or if they are better then the previously mentioned pairs? I have no experience with these....THANKS FOR THE HELP!

(oh and the reciever is a Yamaha RX-V867)