Sansamp Bass Driver DI
I have the Sansamp Bass Driver DI, Sansamp Programmable Bass Driver DI, Sadowsky Outboard Preamp & Aguilar Tone Hammer.
All these have the XLR DI output. I agree with Cherns that the Sansamps will be hard to beat. From my experience, the Sansamp made some of the lousy basses I tried sound really good... can make the bass a lot more alive.
Sadowsky Preamp - I would recommend this to a user with a passive bass... a good passive bass. This give an excellent tight low end... I can describe this as aggressive low end. The thing I noticed about this preamp is when I use this with my bass with a J-Retro turned on... it does not improve or add something major. I noticed this is perfect for a passive bass or with onboard preamps turned off. Roger Sadowsky really found the bass frequency that is huge, tight and aggressive. Comparing to the Sansamp, the Sansamp makes a more dramatical change your tone... this can be good or bad, depending on your taste. The Sadowsky is transparent... while the Sansamp.... hmmm... kinds of give what I could describe like an aura of hugeness... it makes your sound really BIG.
Aguilar Tone Hammer - I find this one very nice for songs that like the warm fat tone. The mid sweep and mid control are very nice and can help you find the mid freq to help you cut thru the mix. Compared to the Sadowsky this is warmer/rounder sounding, I use this mostly on slow songs where a warm bass tone is preferred. This pedal also has the AGS, its like an Overdrive that I never use... I will not comment on it as I never used an overdriven bass to give a decent feedback. Comparing to the Sansamp, both are warm sounding... has an edge due to the midrange sweep and mid control. But when I used them on a lousy bass, the Sansamp really makes the major difference.... it makes your bass sound HUGE.
Most people may not like the Sansamp coz it can the natural sound of your bass
Dont get me wrong... all those 4 preamp/DI boxes are great, thats why I never disposed any of them. All those preamps are versatile for all types of songs... I just happened that I have them so I just choose which preamp to use for certain songs.
Bottomline, I go for the Sansamp for the hugeness/thickness it can give your bass tone. Just start with all the knobs at 12:00 except the blend knob (Full). If you moving the bass, treble and presence with just a hair can really make a difference compared to the aguilar and sadowsky. My Bass setting never reaches 3 o'clock... always 12 to 2:30 only. If on a live situation your bass sounds to Huge or muddy and the bass knob does not help.... try to back off on the Blend Control.
If you want thickening up of your tone and all that, then I think the Sansamp BDDI is hard to beat.
Ignoring technicalities...
Preamp = allows for tone shaping
DI = lets you plug directly into PA system
Poweramp = powers cabinets
Head unit = Preamp + Poweramp combined (sometimes also has DI function)