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before you dig out more cash from your wallet, some tips to get that slap tone.
1. STRINGS
Are you using new roundwound strings? These are the strings which give the most metallic zing when you slap and pop. Make sure that they're new cos over time all your pi sai and dirt will get into the grooves of the strings, making it sound dead and thumpy.
2. EQ on amp
Another way is to adjust the EQ settings on the amp and get a 'scooped' eq setting. Boost the treble and bass eq, while cutting down on the mids. Which means that your dials are in a 'smile' position
3. PICKUPS
What pickups are there on your bass? if there are 2 pickups, i would suggest to only isolate the pickup nearer to the bridge. that usually hits that slap tone.
4. DI BOXES and PEDALS
Of course, if you're intent on getting a box, just get an outboard preamp or DI (like the hartke bass attack) to boost those signals.