Quick clarification

No, as a matter of fact if owners have full range speakers that go down to 25hz or below a sub is actually a bad idea.
Think about it, if you have 2 full range main speakers you could set them to large with no bass limitations, therfore you would get bass in the right side if the material called for it, or left if it was needed, if you throw a sub into it, it is always a mono source. the best way to run movies if you have deep reaching speakers is to set them to large with no sub woofer.
The bass you will get will be just fine as long as you tell your gear the number of speakers you are running.

I would disagree with Chadnliz. Having run my full-range speakers with HT and also with a Sub, the sound is much better with a sub. There are numerous threads here that argue back and forth as to the best way to integrate a subwoofer. My personal experience (and a lot has to do with room dynamics, speaker placement and listening position), the sub crossed over at 80 hz worked better than 2 full-range speakers that go down to 22 hz.

Also the LFE is not a dedicated discrete channel (hence the .1 designation). The LFE will contain deep music as well, the amount of which may depend on the cross over setting. Since you don't have bass management, I'd probably run my main speakers through the cross over in the sub. In this way, you allow the sub's high-pass filter to act as bass management for your mains. Otherwise, you will duplicating the LFE effects (full range speakers PLUS the sub), which will probably be overkill.

As far as I know you can only take advantage of the LFE if your DVD player has a 5.1 procesor built into it, or is connected to some type of 5.1 processor using a digital output. The LFE IS a dedicated discrete channel. It is called .1 because of its limited frequency range. It usually is in the range of 20Hz to 80Hz. Some processors let you set the crossover as high as 200 Hz. The output from the subwoofer in your system will be different than the .1 LFE channel output. If a subwoofer is connected with speaker cables or interconnects from a stereo output you will hear what the crossover wants you to hear. If you use a LFE channel to feed your subwoofer you will hear what the director and sound engineer of the movie wants you to hear.