IMO playing near the bridge doesn't give you more treble, it just cuts some bass to give you more of a mid-punch than a low-thump.
Treble is "click, clack, zip, scrape" etc
I've never played a bass that naturally lacked treble, as IMO this is down to new strings and your technique. I've played lots of basses that seemed incapable of transfering the treble to my rig (it's a bi-amp with guitar amp so i'm pretty sure i'd hear it if it was there).
Personally, for my funky or slap sounds, I prefer to play nearer the neck for plenty of thump, but I add in plenty of treble using my pedals and amp.