To slap you need high action. As a matter of fact real low action does not suit the double bass at all, in any style. Bear in mind that the action at the fingerboard is only of very relative significance and it all depends on hight at nut, string type, string tension, bridge curvature, in short general bass set-up as well as your technique and stamina. To over-simplify and answer your question: some people can slap steel strings with low action around 7-8mm. This low action would not suit wackers or guts as you will have no acoustic volume. Personally I use gut strings and action around 1.2-1.3cm. Very high for most people, but I like it and I can be heard even without an amplifier (up to a point of course). That's how the old players did it, since you asked about classic slap tone. Some modern psychobilly guys have different setups which rely on amps a lot more, but I really don't know about that. Gee that's a long answer sorry my friend.