Install, speaker placement, and EQ can make a world of difference, but to be honest, the RSD's, while great for the money, are really a budget speaker and don't have the detail of some others. I believe the RSD's are metal plated poly... which is probably no better than a poly cone, many of which can sound a little lifeless. The poly cones are often over-damped, and even though the metal is not, the way the metal is applied can add even more damping.
After you exhaust all install/speaker placement and tunning issues, I would recommend trying different speakers. You should be able to sell the RSD's for a good price right now. Cerwin Vega V-max 65 from sonicelectronics.com would be a good set to try that won't break the bank.