Cross over frequency ???

Several factors come into play here.

1) What type of speaker are you using for the mains i.e. vented or sealed ?

2) Are you running the mains full range or rolling them off ? If rolling them, are you doing this actively or passively ?

3) How much overlap is there between the mains and the sub i.e. what freq are the mains crossed at if crossed at all ?

4) Where is the sub placed in the room ?

5) What is the proximity of the sub to the mains ?

6) What does your in-room frequency response look like without the subs energized ?

Lot of questions here, but there are a lot of variables involved.

As a general rule, if you are running the subs to supplement the bottom end of a large full range speaker, cross it over as low as possible. If using the sub to take over the bass chores of a smaller speaker, you want to cross the smaller speaker over at a point that is slightly above resonance and bring the subs in at a point that is slightly below that resonance. In other words, if the smaller mains ( aka "monitors" ) resonate at 65 Hz, cross them at 80 Hz and bring the sub in at about 45 or so.

While one would think that there would be a gap there due to neither of the woofers directly covering the range between 45 - 80 Hz, what happens is that the lower output from the sub and the mains sum together to create a relatively even balance. If you don't create a "gap", you'll end up with too much overlap i.e. where both drivers are covering the same range, producing too much output in that region. The end result is bass that sounds very "one note", slow and sluggish.

Obviously, this was just a "generic" description. What frequencies you should use and the amount of "gap" that you should create will vary with the crossover slopes and speakers being used. The only time that one would not want to use such an approach is if you were using "brick wall" type slopes. While some may consider 24 dB slopes relatively steep, you'll still have a considerable amount of "blending" between drivers taking place. In order to minimize blending / driver overlap, you have to have REALLY sharp slopes. Going active is really the only way to achieve good results doing this in my opinion. I'm sure that Jeff Joseph would argue this point with me though : ) Sean
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I think you laid it out very well Sean. Just one thing added, if you use something like a 6 db slope the transition will be very smooth but you may find out that you may negate part of the effect of the subs because the satellites are still active for too long and the sub sounds muddy because it extends too high. As Sean said, then you definitely want the crossover point to be different between the subs and the satellites.

In my system the processor does both the high pass and low pass.Your advice if I understand it correctly is to use the processor to xover the ported monitors and a seperate xover to xover the sub ? I use Paradigm Mini MK3's as mains and center.The extra goes to the ES ch.The subs both sit in the same corner stacked.Funky stuff happens when I try to seperate them.The manual for my EAD say it uses a 24db/oct linkwitz riley.Seems steep enough to do the job ? The room has a very robust sound with films.I am very happy with it but my mini's tend to not like the lows of HT.80hz just didn't cut it.Oh well.

No, not two separate crossovers just different frequencies for each section. If you could you would leave the subs at 80 hz but crossover the monitors at lets say 100hz. Twenty -four db is pretty steep. That should be a fast enough slope. If your monitors don't like 80 hz try raising the crossver gradually upward. The higher the bass the more likely it is that your sound will get muddy.