The preferred way is to connect a cable from the receiver's sub pre-out to the L(eft) line level input on the sub. This connection allows you to use the receiver's bass managment capabilities. You then have two choices for the LFE switch on the back:
1. Set the LFE switch on the sub to LFE. In this setup, the sub's internal cross over is disabled, so the position of the dial on the back of the sub is irrelevant.
2. Set LFE switch to 'normal'. In this setup, the sub's cross over is not disabled and you have two competing cross overs (the receiver and the sub). You can do this and turn the sub's crossover dial to a setting higher than the receiver's to get it out of the way. It's better to just use the LFE setting and rely on the receiver bass managment, but some people might set it to normal and fiddle with the two cross overs to achieve a good blend.
As far as the correct cross over setting, a general rule of thumb is 1/2 to 1 octave above the F3 point of the least capable speaker. The F3 point is the frequency where the speakers output is down 3 dB and can be found in the specs for the speakers where it might say something like '75Hz - 20kHz (+/- 3 db)'. An appropriate setting would be anywhere from 80Hz (right near the F3) to 120 Hz (half an octave above 80Hz).
All speakers should be set to 'small' for the above. Speakers set to small will have all bass below the cross over point you set, sent to the sub. LFE, if present, will be sent to the sub only.
Speakers set to 'large' will get a full frequency signal and any bass destined for that channel will NOT be re-directed to the sub. Only the LFE, if present, will get sent to the sub.
Settings called BOTH or LFE+MAIN will send the bass and LFE to speakers designated 'large' AND the sub. Only do this if you have very large main speakers that can play very low and/or you crave alot of bass - but beware that the bass may be too overpowering using that setting.
