Setting LPF

Right ... but you should still have some sort of crossover on your other speakers.  What size speakers do you have up front?  You should follow the guideline that francious gave you.

/r7, no, crossing the sub over that high probably isn't a good idea.  First, you should know that a crossover doens't just stop at the crossover frequency.  It'll still be playing octave(s) above the xo frequency depending on the xo slope.  Many car audio subs will encounter breakup (some crazy freq. response) if crossed too high.  Second, bass is easier to locate the higher it is crossed over.  Third, by asking the sub to do more, you're taking power away from what it does well.  Fourth, his subs are probably located far away from his mids and highs.  As the wave lengths decrease at higher frequencies he'll start encountering more phase related cancellation effects. 

I'd keep the sub crossed over at 100hz or below if at all possible.

A good 6.5" midbass driver can be crossed over successfully at 100 or 80hz with a 2nd order or higher xo.  You're right though, this can stress the midrange... this is why some people upgrade to 3-way systems.  :-)

Here's an excellent page on http://www.bcae1.com/ explaining this... http://www.bcae1.com/xovrslop.htm

Yes, generally whenever a speaker is asked to play less bandwidth near its limits it can perform better.  I recommend what I do because a decent 6.5" midrange can handle a 80-100hz xo better than a 10"+ car audio subwoofer can generally play with a 150hz xo.