Jeff Zamek did a nice paper on s-cracks: his conclusion is that it is all about the power wedge aka: cone up and pushing down phase.
Perhaps with these pieces you are using more clay and therefore not engaging the clay enough at that stage.
Try really making sure you are involving all the clay down low in the cone. Really push it together at the bottom. If you have a wide cone then the bottom clay is not getting involved in the power wedge.
Cone up and push down a few more times than normal and see if that solves ( in combination with the normal compressing the bottom tricks of course).
I have not had a single s crack since paying heed to this advice (knocking vigorously on wood:)
... and you can even cut this well-worked cone of clay off the wheel and flip it over and do it again. This way what will become the bottom of the pot was once part of the most-worked part of the above described cone.