Bring your string height down as low as it can go without any fret buzz. Neck must also be adjusted (it's gotta be straight). Slap sounds best with Fender style single coil pickups but it really doesn't matter. Basses with humbuckers (MTD, Smiths, Peaveys, etc) sound really good when slapped. Basses with Musicman style pickups (Stingray, Laklands) also sound great. It doesn't matter.