The 80s can play down to 40 hz with ease so a I would recommend 40 or 60 hz as the Xover point. I run mine at 40hz and have had them at crossed over at 60. Experiment a bit, try 40 then 60 and then back to 80 and decide which you like better and go with it.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70