Trust me, you don't need to buy anything if you have Sonar Platinum.
First off, use the big Tube knob in the ProChannel on bass, kick/snare/overheads/hat and perhaps even on the guitars. Secondly, use the Tape saturation on the drums, especially on a master buss or on the overheads. It works GREAT for softening harshness. Make sure you don't have too much high end frequency stuff going on (roll off some highs on guitars!). Put a console emulator on individual tracks and busses.
If you have some money to spend: get the Cakewalk CA-2A. It's been sad it's not a particularly exact emulation of the LA-2A, but I think it's awesome! For Kicks and Snares it make life so easy. Super warm, nicely controlled...
R