Probably the most underrated and under-appreciated pedals out there are EQ pedals, or really any equalizer set up in your chain. They are so simple yet can be the ultimate tone fixer. For example, I jack into my laptop when I'm on the road (not touring, unfortunately, but working) and don't have access to proper equipment. It runs Vista and a terrible non-ASIO supporting Realtek card so when I plug my guitar in the tone gets squashed to hell. Since the software support for the hardware is terrible (my old-ass Soundblaster card from the 90s and Windows '95 had more options) the only thing I could change was what went in to the computer. Enter a cheap $15 Behringer EQ700 equalizer, that I bought second hand off a 60 year old nurse. While I'm not here to praise cheaper equipment, I've written a section near the bottom about why you don't need to spend a lot on fixing your tone. Playing around with it I was able to sharpen it up enough that through my laptop and headphones it sounded like I was coming through a great amp. Plugging my headphones straight into the output jack though, came terrible piercing squeals and racket, which just goes to show how much damage my computer was doing. But nonetheless, the EQ pedal saved it and that's what they're for.
Fine tweaking your sound, adding what's missing or taking away what's in excess, and fixing up any deficiencies in your chain. I'm going to assume you all know what equalizers are and what they do. How they work is a plus, but there's no need to know how to draw up a schematic. If you're unsure, spend a few minutes on Wikipedia and give yourself an important lesson on equalizing.
Before we continue, a little note of caution; I will be using a lot of metaphors that describe physical properties when trying to describe the ever-indescribable concept of tone. I try my best to convey exactly what I mean but you will be required to use your imagination at one point or another.
As with all pedals in a daisy chain, one of the foremost things is where in the chain it is. Most effects usually have two main options before or after drive. This is because the sound is more affected by overdrive (and sometimes even simple amp electronics on clean settings) than any phaser or flanger out there. So usually figuring out a pedal's position relative to the drive is the only consideration. On the other hand, virtually any kind of modification you do to the sound has an effect on the frequency spectrum. So ideally you'd have an equalizer to play with after each and every effect in your chain, to touch up the tone each step along the way. A lot of systems do offer that option, somewhat, in the form of a single knob dastardly labeled as tone.' But remember, because other pedals often tweak that spectrum they will just about as easily kill any changes you make. Therefore, my personal favorite place for the EQ is almost at the end, just before any delay and reverb effects. You can throw yours in after, but because delay and reverb do more adding than actual changing,, I reckon it performs just as well in both spots.
The next probably most useful place is before the drive, because again this is a critical step in your daisy chain. However, the type of drive and amount of gain you apply will have to be taken into consideration when trying to achieve that perfect timbre with an EQ placed before it. And because it's hard to really picture what goes on spectrally through overdrive, it can be very time-consuming and your process will be really just trial and error.
Starting with the sliders or knobs all at zero is important you want to be working with the tone you have, and coloring it (or discoloring it) from the start is not the way to go. It's almost like modding a guitar. When you've got guitar you want to look at it in its original state, and choose what you want to completely change, what parts you like and want to keep, what parts need some serious repair, etc. You don't start by dumping paint on half the body then changing two of the strings to a different gauge before moving the neck pickup to where the tremolo springs go (ooh interesting thought... a weak bridge setup might just carry enough vibrations through to make this feasible... any takers?). Take what you have and work with it.
Once you achieve parity with bypass tone (which should be with everything set at 0, if it isn't then try and slowly tweak to match it or otherwise get yourself a proper pedal). Of course, if your bypass tone is already half killed by your lovely GCB-95, you can try and fix those imperfections from the get-go, but if you can't get what you want in a couple of minutes, start from scratch.
I always find that starting off in the lowest end of things helps. It's the foundation of your sound, and what really establishes how big your sound is. Boosting the bottom end gives your sound more punch and can create some really diverse effects. I'm referring to, of course, resonance frequencies. Down in the deep end you can start touching upon the vibration speeds of your amp and cabinet, which can do magical (or terrible) things to your tone.
Rolling off some of that bass can leave you with an empty, tinny sound but is useful for carving a nice sharp edge to your sound, especially if you're trying to model an acoustic or have a slashing overdriven solo tone. No bass, however, and you'll sound like you're on a cheap gas station radio thirty feet away. Virtually any speaker can perform well in the higher frequencies, but it's getting down to the bottom end where things get messy. You may not even notice much change when changing anything under 100 Hz. Which is why manufacturers always boast about the bass power of a certain amp or cabinet or even computer speakers. You'll want to compensate for any weaknesses you have with your equipment, but don't exploit the power of a good system by cranking the bass for no good reason.
Now that you've got the foundation of your sound ready, it's a good idea to tackle the high end. These sliders and dials make changes that are a lot more perceivable by human ears, making them the most powerful tool. This is also why guitars given tone' knobs usually just are treble cutters. Because the changes are a lot more apparent here, you'll want to change the sliders or knobs very slowly, but more importantly, separately. Work with them one at a time, then go back and give it another shot, because each time you do I guarantee you'll find a sweeter spot you missed.
The treble often constitutes the crispy, biting end of a crunchy sound. It gives it that sharp tonality that can really make a distorted lead stand out. Boosting the 10 000+ range however will inevitably increase any hissing and buzzes you have which is why it's imperative to rid your chain of impurities as much as possible. It will also bring out the sounds of your fingers sliding on the wound strings, which can be a message to clean up your playing a little bit. Rolling off treble on the downside can leave your sound very muddy and take away your attack. Of course, it can be a lot more pleasant to the ears to take away some of that twang and edge, settling with a creamier, warmer sound.
Often clunked together, I like to take an extra step and divide them up into two more groups the low-mids and the high-mids. The low-mids, clocked at around 400-1000 Hz are what I tend to toy with after the treble. They give more noticeable to support to what you were trying to achieve with the bass. Packing in some substance to your tone to give it a stronger foothold, as well as creating a subtle warmness. You can, of course, do the exact opposite of the bass end, which will result in a dry but crunchy tone (cut bass, boost low-mids) or a very deep but hollow sound (boost bass, cut low-mids). Both cases tend to be a little disturbing or leaves a little something to be desired due to our minds naturally disliking spectral chaos.
Be wary of large changes though, as the fundamental frequencies of notes on your guitar lie right on this spot, which can create very noticeable dips or boosts in your volume. Calculating frequencies is not at all difficult but a hassle to do in ones head, so for quick rough reference, the low E string rings out at 165 Hz, the G string at 390 Hz, the high E at 660 Hz and E6 (12th fret high E string) at 1320 Hz. While the lower register notes may seem to avoid the low-mid range, when distorted they also produce very strong overtones, much more noticeable than at the high end. Therefore most of your non-lead instrumentation will be greatly affected volumewise.
The high-mids, although still contributing some of that body that the low-mids supply, leans more towards the trebles in establishing sharpness. Turning them up can make your sound feel very alive yet solid. Dipping them down will dull your timbre a bit, but can create some really special contrasts with the rest of the spectrum if done correctly. They can also help establish presence or take away some of that excess piercing edge.
That Last Slider
Almost all equalizers will have one last slider, that works more or less like a preamp. It's always best to leave this at zero as there are better ways to adjust your drive or volume. Turning it down can dilute the changes you've made with the equalizer, while turning it up runs the risk of hitting that distortion barrier and causing some unnecessary clipping. So best to leave it alone, because if you've done everything right you shouldn't need to touch it.
The great thing about EQ pedals is that their process and internals are so simple and so defined that virtually all of them are of equal quality. Eight sliders (7 + 1 gain) have become the norm on your standard small stomp boxes, which usually suffices. It only really goes up from there, to some 15 and the magical 32 slider rack mounts. Remember, three knobs labeled bass,' mid,' and treble' do NOT constitute an equalizer. Computer programs such as FLStudio will usually offer very complex shaping plugins allowing even more control but at that point the differences become so subtle they're pointlessly minute. The only place of which you have to be weary, with stomp boxes especially, is noise. Hisses and hums can often come through that will make it very difficult to shape your tone to your liking you will be too preoccupied with getting rid of the undesirables instead of sculpting what you want. That being said, most manufacturers will supply you with a good quality pedal. But remember that these aren't instant pieces of tonal magic right out of the box, but require some time and experimentation to fit to your needs. So try and test out a rig as much as you can before putting down any money.
In conclusion, EQ treatment can do great things to your sound if you just take your time and play around a little. Don't be afraid to experiment. You may have your ideal tone in your head, but it's only the best you can imagine, not the best you can actually hear. Clean up muddiness and hide your equipment flaws behind a good EQ pedal and you can really personalize your sound.