I almost always am searching for that tone I hear in my head and therefore usually mod to some degree. Whether it be fitting the tone that naturally comes out of the bass more so to what elements will bring that out more, or changing pick-ups to emphasis other characteristics that are absent.
Dimarzios make a passive Warwick truly sing. Though I do have one bass I am never modifying; surprisingly its a cheap (ish) Korean Schecter, but it does one thing really well as is. Mwah for days and very even tone across the spectrum no matter neck or bridge pick-up selection. I love that bass.
On the other side of the spectrum I have a MIM jazz that, except for the wood and frets, nothing is original - It is a true beast now. Very bright and angular - Very Geddy. It's pretty heavy wood for a jazz, so I'm not surprised that I could unearth this tone, but it didn't start out sounding this way at all.
And then sometimes what started out as a small mod turns into a very large list of mods - trying different pick-up, preamp combos, changing the bridge to get more sustain, string changes to mellower or brighter, adjusting the action to get more even-ness throughout the fretboard/strings, etc. Sometimes some of this experimentation is unsuccessful, but I think learning from it all is pretty paramount to the next attempt.