The clipping has to be happening at the master bus or later, since the Session Drummer samples would not normally be clipped at the default volume settings.
First, make sure everything's routed through the master bus. Mute the master and all should go silent. If it doesn't, then there's a routing problem.
Next, determine whether the signal is clipped at the master bus. The meter should not hit the red. You can also turn on wave preview on the master, to visually check for flat-topping.
If there is no obvious clipping at the master bus, export the song and copy it to another, totally separate, playback device such as an iPod. Or burn a CD and take it out to the car. If there is no distortion, then the issue lies with your monitoring.
The most likely culprit would be your interface settings. Second-most likely is the interface itself. Fortunately, the least-most likely suspect are your powered speakers.