I'm not as good in slapping as the others here but i'll share the little i know. Since having my MXR DI i normally set the eq of my bass amp to flat. or 0 0 0 0 for bass to mids...if its one of those knob like eqs then i'll prob set bass to treble at 5 or something.

the actual tone setting is done at me DI's eq. For my slap tone....i prefer a more "oingo-Boingo" (sorry for the wierd description) sound...a springy bouncy tone. bass and trebles would be boosted while the mids lowered to emphasise the low slaps and sharp pulls. Minor fine tuning are normally done on me bass guitar.

Sometimes if i'm in a jam studio are just having fun with friends I just use which ever bass setting that the previous bass player used and adjust accordingly with my fingers

hope this helps