Here I'll try to clear up a few common questions about adding an extra EQ device to your rig:
Can I get more low end out of an overdrive/distortion/fuzz pedal by adding an EQ pedal?
Can I get more low end out of my practice amp by adding an EQ pedal?
Can I get a more modern, clean, crisp tone out of my amp by adding an EQ or a preamp?
My amp already has EQ, will there be a benefit to also getting an EQ pedal?
My bass has a preamp with EQ, but I see lots of pedal preamps with EQ, should I get one?
Would a rackmount graphic EQ be a good addition to my amp?
For all of these questions, this is the first and most important answer:
Think of EQ like a lens from a pair of sunglasses. Different lenses can have different color, darkness, polarizing, prescription, and so on. You can put two different lenses one on top of the other, and it will get you a new overall result, but it's not like one lens cancels the other one out! If a lens is too dark, putting a lighter lens on top won't lighten it. If a lens is scratched, putting a new lens on top doesn't fix the old one. With prescription lenses, putting the "right" prescription on top of the "wrong" prescription just results in a blurry mess.
It's exactly the same with EQ. Adding an EQ can't make your amp or effects pedal have deeper lows or higher highs, and it can't make a muddy amp sound more crisp and clear. EQ can only take what you put in, and reshape it--EQ cannot "put back" frequencies that were cut off.
Now, in practical terms that may not always matter. For example, say you have a pedal that cuts off the lows below 80 Hz. Running it into an EQ set to strongly boost the lows may make it seem like the low end is fully back, because of the way our ears and brains work: with enough upper harmonic content from each note, our brains will fill in the missing lower harmonics, so we will actually hear frequencies that are not there. The same thing happens with our speaker cabs--most of them can't reproduce the lowest frequencies we play, but we hear the low notes anyway just because of a trick of the mind. Plus, what we often think of as "huge booming low end" is usually a big hump peaking somewhere around 80 to 120 Hz. Either way though, the important thing is just to be conscious of what you're actually getting from an EQ, so you can use it most effectively.
It's a similar situation when trying to get a clear, crisp, "modern" tone. You can't make a dirty window clean by putting a clean window on top of it! There are gadgets for getting a brighter sound, like the Aphex Aural Exciter and BBE Sonic Maximizer processors; but generally, all they're really doing is turning up the highs. I guess you could say that's like shining a flashlight through a dirty window. Goofy similies aside, the fact is your amp can only do what it's built to do, and adding an extra EQ won't make it perform better, and won't eliminate the coloration the amp adds to your signal.
Lots of people get the idea that if their amp has frustratingly limited EQ controls, they should get a rackmount 15-band or 30-band graphic EQ for it. When you are just starting out, this is might actually be a good idea for an at-home educational tool so you can learn what results you get by boosting or cutting various frequencies. Graphic EQ helps you visualize those changes, and associate the visual with the sound. But when you are a more experienced player, and already have a good understanding of EQ shaping, then you will probably find the graphic EQ is not precise enough, transparent enough, or small enough to be worth keeping in your rig. A parametric or semi-parametric EQ can give you better results, with fewer controls. That's why most professional amp heads made today use those instead of graphic EQ.
As far as pedal preamps, most of them don't have any better or different EQ than what's already in most amp heads and most active instruments. It makes sense to get a preamp pedal if it offers features or tones you don't already have. Just because a hundred people on an internet forum rave about how awesome a certain preamp pedal sounds, doesn't mean it will necessarily do any good in your rig. Remember too that the more preamps and EQ's you put in the signal chain, the more your signal gets degraded, like many lenses all stacked up. The best instrument tone comes from the cleanest, most minimal signal path you can manage, while still getting the tone-shaping you want. Albert Einstein said: "everything should be made as simple as possible, but not simpler."