It's hard to find another brand of replacement pickups for the $$, but first I would look for other ways to improve the tone - either string height, new/different brand/type strings (probably your Number one fix), or even technique changes to accomodate the larger pickups.
I have a 5-string $$ and find it sounds great when slapping. I would seriously look at new strings (I get great tone from my Warwick Black Lable steels, but a lot of people here like DR High Beam nickels) and changing the string height. For a fatter, more resonant tone, try raising the strings to give them more room to "bounce" and ring.
Don't give up on the $$ - it's a very versatile and capable bass. You made a wise decision to buy it. But it is a different beast to get used to and once you do, you'll fall in love. Trust me!