It's taken me a LONG time (4+ years) to figure it out but the slap tones that I like seem to only come from long and extra long scale basses with that have 22 frets or less (with the exception of the original Yamaha TRBs! I'd heard the theory before but never really paid it any mind until now. Until fairly recently I only wanted basses with 24 frets and when I finally decided that 24 wasn't necessary for me I found the tone tone that I dig the most (jazz and PJ basses). There might be some 24 fret basses out there that can do it but they seem to be in the minority, though I did hear some Warwick or another cop the MM tone perfectly. Just thought I'd share my findings .