A headphone amp so no-one else has to hear it.
+1 Now that response I like.
In all seriousness though, I wouldn't purchase an amp based on one particular technique. It might sound great for slap but you may find it sounds horrible when playing fundamental finger style which you would be playing for 90% of the time in 90% of bands.
Look for one with a great basic all round tone that is flexible enough to dial in a great slap tone when needed.