Mine are Hertz HSK 165s. They're at 80 right now. I know for a fact they can just as easily handle 70HZ or so for pretty much any kind of music but my rear fill isn't amplified, therefore lacks its own crossover. So the simplist solution to keep everything clean for the time being was to cross everything at 80, the lowest setting on my HU.
Eventually I have to figure out whether to either eliminate the rear fill or find a way to cross it over separately about 80 HZ by itself so I can put my fronts closer to 70. I would never go below 70 on most midbass/midranges, you start to loose too much volume if you play bass heavy music.