When I just want to use a 100 watt stereo receiver to power a subwoofer and two speakers, thus a 2.1 setup, do i need to use the amp built into the subwoofer or can i just use the crossover in the sub with the line level inputs and outputs and have the receiver's volume power the sub and speakers together?
Even if you connect the sub at the speaker level, the sub amp will power the sub. The signal will be attenated internally and re-amplified, but you don't have to do anything other than run speaker wire to the sub. You can either "daisy chain" it which is to run wires from the receiver to the sub and from the sub to the speakers. Or you can connect your speakers to the "A" terminals of the receiver and the sub to the "B" terminals provided the A & B terminals are in paralell when both outputs are on.
If your receiver can amplify the left and right mains and simultaneously provide a line or aux output, then just connect that output to the sub, and the sub will use its own inbuilt amp and crossover filter. Then the main volume should be controlling the amount going to the sub anyway through that line output. The stereo receiver/amp will power your main left/right speakers, but not the sub. It's a stereo amp, which is 2 channel, so it doesn't have a third powered (amplified) output for the sub, so you'll want the sub to use its own.You just want to put it on a connection controlled by the volume knob. Then the volume on the sub can be used to control the ratio of sub to mains level.
You could use a passive sub with its own passive stereo crossover for the mains driven by the receiver/amp, if you had one, but your sub is active and doesn't work that way.