I really, really don't like onboard equalizers on bass. I don't mind preamps themselves so much, but I just don't like the EQ controls.
Why?
Reason #1: Most onboard EQ is very limited. Most preamps have shelving EQ. You turn the bass knob up and ALL the bass frequencies get louder. You turn it down and they ALL get quieter. Same goes for treble. I like to control exactly which groups of frequencies get louder/quieter and by how much. You could just say "why not bypass the EQ?" This may come as a surprise to some of you, but most active basses out there don't have bypass switches. Also, while you can just run them flat, you can't always do so very easily.
An example would be the "style sweeper" on the Ibanez 300 series soundgears. It either scoops the mids (pretty severely) when set to "slap", or, it boosts them slightly when set to "finger". In order to set it "flat", you have to turn the knob to about "3". Which brings me to my next point:
Reason #2: I, personally, do not ever use EQ knobs on a bass in an "inbetween" setting. Meaning, I either have them at 0/1 or 10. Why? Because I don't want them to get moved when I've found the best spot for them. Let's say I really liked my bass knob at about six. Then I'm playing and my hand slips or I bump into something and the knob gets spun out of whack. I don't want that to happen! So, I tend to just min/max with EQ knobs on the bass. Also, I don't want to have to remember exactly where to set the knob for it to sound the best.
Reason #3: They change the total output level of your bass. I run sound at my church and it's a pain when the guitar players switch pedals and their volume is different. It just makes mixing that much harder. I like my output level to be the same no matter what. At least from my bass itself. That's why I have my volume knob at 10 all of the time. I don't want my bass to suddenly be louder because I boosted the bass knob. I don't mind if my "end sound", the sound heard by the audience, changes. It's the signal level going to the rest of my equipment that I want to have remain constant.
Reason #4: It's just too complicated. I'm a set it and forget it kind of guy. I find one tone that I like and use it exclusively. Some people like the versatility. That's great for them, but not for me. Give me a bass with one knob, I'm happy.
/end rant