Well, it all depends upon what you want the box to do. For example the SansAmp gives you a warm sound and it emulates many famous tube amps, the Ampeg SVT, Fender Bassman etc. Then it can also be used without those effects. Some say the tone is colored though.
By all means check out the links which Munjibunga wrote up earlier, there you can hopefully find what each box does. There you should be able to find what you want.
I only know the SansAmp so I can only comment on that one, maybe owners of the other boxes can comment on what they´re like.
Here´s what the Tech21 site has to say about the SansAmp.
SansAmp Bass Driver DI gives you the warmth, presence and punch of a miked-up bass tube amp system in a conveniently portable direct box/pedal format. SansAmp Bass Driver DI can be used as a pre-amp or stomp box with your existing stage rig. You can drive a power amp and speaker cabinets. Or go direct into the mixer of a recorder or P.A. You can also plug into a power amp or conventional bass amp and a P.A. system simultaneously.
The controls offer a broad range of traditional bass amp sounds, including such meaty styles as Bassman® and SVT®, as well as raunchy, crunchy overdriven sounds that would typically require a multitude of effects units.
PRESENCE controls the amount of definition and upper harmonic content.
DRIVE adjusts the overall gain structure and overdrive.
LEVEL regulates the XLR and effected 1/4" output levels.
BLEND allows you to mix the direct bass signal with SansAmp Tube Amplifier Emulation circuitry (which is particularly useful for basses equipped with piezo pickups).
BASS & TREBLE are active EQ controls, specifically tuned for bass guitar, that cut or boost +/- 12dB.
For studio and live applications, SansAmp Bass Driver DI is engineered with three outputs: an effected XLR, and effected 1/4", and a parallel, uneffected 1/4". The footswitch activates the Tube Amplifier Emulation circuitry. Disengaging the circuitry enables you to use SansAmp Bass Driver DI as a standard active transparent direct box.
Sample style settings include Bassman®, SVT®, King's X, Slap, Reggae, Crimson. Settings are also included for Chapman Stick and Acoustic guitar.