the preamp in it's strictest sense is just to take your low signal output from your guitar and amplify it to a level that your amplifier can handle better.
In your bass amp, it will include the pre-amp and the power amp. the pre-amp will amplify your low output signal to a higher output and feed it to the power amp which will then amplify it even further to louder.
but normally, pre-amp also include some tone voicing, coloring functions - which include a EQ, and other mojo circuits added to give it "that" sound.
the hartke bass attack has a EQ part - bass, treble and "brite", as well as coloring with the "harmonics" knob. The shape knob is a parametric notch filter, to my ears, i think this makes the most difference in the sound.
other pre-amp pedals made by different manufactures have different ways of "coloring" the sound, the inherent voicing and coloring should already be built-in and they mostly do include a EQ for you to tweak it to your own taste.