Several question about the set up.
1. What is your source material - i.e. are you playing something with a lot of bass or very little bass?
2. Does the DVD player have a digital audio output?
3. Does the receiver have a digital audio input?
4. Does and DVD player have 6 channel analog audio out?
5. Does the receiver have 6 channel analog audio inputs?
6. Does the receiver have subwoofer preout?
If yes to #2 & #3, are you using that connection?
If yes to #4 & #5, are you using those connections? And do you have the receiver set to multi-channel input?
Are the front speakers set to small or if set to large LFE must be set to both?
If there is no sub pre-out, you need to connect the sub to both the left and right front speaker outs from the receiver, and connect the speakers to the speaker outs on the sub. The sub will do the crossover.