You're missing metal on metal.
there are a lot of folks who are quick to say there's no problem with slapping a fretless, but I'm not one of them for several reasons.
1) The smiley face EQ curve you like for slapping cuts the heart and and soul out of what make a fretless sound great - gobs of midrange.
2) Slap playing is all about attack. Fretless bass by nature has a softer attack, due to aforementioned lack of metal on metal.
3) All fretless players use their ears to microtune as there are an infinite number of bad notes on a fretless. Since slapping is all about the attack, if your intonation isn't
dead on the note pops out before you can do anything about it.
These are just my observations, but they come from having been gigging a fretless for many, many years. If you're slap-happy, you're much better served with a fretted bass.