Subwoofer xover settings for KEF LS50W (orig model)

i run all program to my sub which paasses everything above 80Hz to the bookshelf speakers which are run full range

Always high pass the speakers if possible. Once you high pass you can set the crossover anywhere you want and it will sound fine. 
A good spot is where the driver starts to take a dive and it would be crossed over to the next larger driver if there were one (in this case where the port pics up). The driver is down 5db by 70 hz on the ls50 but has flat output to 100. Somewhere between 70-100 is the right spot. Just have to see if how the quality of the sub is. I have large floor standing speakers with 8” drivers and cross over at 60z. I don’t here much difference going from 50-70 with a high pass.

With just a low pass it sounds pretty bad to me. Timing is off and it sounds best with the low pass just above my speakers lowest output (low passed at 40z and my speakers go to about 30hz) with a 24db roll off. 

Just a follow up. I set the high pass at 70hz and low pass at 60hz after much listening and measuring (test CD&SPL meter). The ls50w goes to lower frequencies than the passive ls50s. Confirmed with measurements. I had excess dB right around 60-70hz and ended up with the above settings to help best I could. Thx for the advice here. 

I hear 80 more than anything else and overlap the low and high settings because of the way the filters roll off instead of hitting a brick wall. 

Jbs,

good deal. How do you like the results? Happy with the sub as a whole? I am a big fan of subs and will probably never have a system without them. 
Do you know the slopes of the crossovers you use? 12 and 24db slopes need different overlap points. I am stuck with the single 24db for both low and high pass but it works out well for me as is.