I'll hop on the outboard preamp bandwagon.
I've got a SB-2. I replaced the bridge pickup with a Nordstrand dual coil pickup, but that really put it the other side of warm and round - which is what I wanted.
When I want warm and round, I'll one these things:
Turn on the SIB Fatdrive. Nice unit - warm, round, tubey.
Change channels on the Bassbone to slight mid scoop plus the SIB Fatdrive. Warmer, rounder, fatter.
Crank the gain on the EHX Blackfinger in addition to the above. Starting to get into overdrive territory.
Or, turn on the OD pedal with one or more of the above.
One sans amp model allows you to program in three different settings (the BDDI?) I owned an Aguilar DB924 - very nice unit! The Sadowsky preamp pedal is raved about, too. It's got something they call a Vintage Tone Circuit that apparently is "the bomb!"
The nice thing about the pedal option is that it's really easy to experiment with and you can use them on all of your basses.
Another thing to consider is the varying levels of power in the preamp section of bass amps. The EQ section on one of my amps really doesn't change the tone that much even when cranked one way or another. On another amp, changing settings even 1 or 2 makes a HUGE difference. The EQ section of the sans amp, Fatdrive, Bassbone, and Aggie units are more like the second amp I mentioned.
hth
I find it hard to resist the urge to mess with pickups and onboard preamps on basses. I've done it three times and only been happy one of those times - lot's of labor $s down the tubes on mods. If you don't like a pedal, you can easily sell it, esp. those listed so far.