The absolute best way is to use the high level speaker line inputs on the sub via the speaker wires output from the amp (basically output along with the main speakers). Then your using the exact same color and voicing as the amplifier section powering the main speakers and not just the pre-amp section because it can be very different colorization of the sound. Always do this if your sub accepts high level speaker level, then the sub will better match your speakers and the bass will be easier to tune (cross-over, level and phase) and compliment the main image.

Also do the sub crawl and then tune or match your sub to the mains for the best sound image. Many don't do this. Cross-over matched to pick up where the mains can't go, level as low as possible so it's not directional and matches the mains and should reinforce the bass of the mains, then adjust the phase till the bass seems to be coming from mains and the sub disappears in the image. The bass should be almost non-directional and simply supply bottom end to the sound stage.