I don't actually own an Axe yet.. what I've described applies to pretty much all multi-fx units that have a good level of controllability..
the GEQ and PEQ are very different animals.. again this is not specific to the Axe.. most reasonable quality fx-units will contain blocks that offer the same functionality..
Graphic EQ - this has multiple frequency bands that are fixed
if your guitar amp has 4 tone controls - bass, mid, treble, presence - you can think of this as being a 4 band GEQ..
each control effects a pretty wide range of frequencies... the 'width' [the frequencies it effects from lowest to highest] of the control is called the Q..
in a GEQ you can't change this.. an 8 band GEQ is more of the same but by having more bands you have more detailed control and each band's Q is therefore narrower..
think of an 8 band GEQ as having low bass, mid bass, high bass, low mid, mid, high mid, treble, presence
and likewise a 16 band GEQ would have even more controls with narrower Q values that sit in between those...
this being the case.. a GEQ is pretty simple to use..
a Parametric EQ is far more flexible but there are a few things you need to understand to get the best out of it..
each band has a Q control.. so you can change the width of the band.. low numbers are a wide EQ range and high numbers are very narrow..
this means that you can have very general EQ control with values from 1.0 or lower.. or make precise changes [usually corrective] with narrow values such as 1.8 and above..
The frequency control allows you to change 'where' in the frequency spectrum the band acts upon.. so unlike a GEQ, a PEQ band can effectively be moved up and down the frequency spectrum..
the first and last bands can also have their filter type altered.. whereas the inner bands use a 'bell shaped curve' to effect the frequency band.. the outer bands van be bell curves, hi-pass [let hi freq pass], low-pass [let low freqs pass] etc.. this allows yet more refinement..
frequencies important for guitar...
a guitar is actually a tenor instrument [despite all our music being written with a treble clef] so in general terms we can get pretty low [concert tuned]
our low E is just a little higher than the low C of a cello [which is a bass instrument]
the numbers I'm using here are just a rough guide
sub bass - this is below 80Hz.. guitars actually produce sounds lower than their lowest notes via sub-harmonics etc..
this can cause a loss of clarity.. many produces will cut everything below 50Hz to tighten the tone
bass - 100Hz to 400Hz this is where all the power is..
but too much will destroy your definition and make you sound woolly
low mids - 400Hz to 800Hz this is where your thump is..
if you want deep bass then scoop these... but beware.. a lack of these will get you lost in the mix..
the relationship between bass and low mids is critical
never los sight of the fact that your bassist provides the deep bass.. so often you can get away with less bass and more low mid, and between the two of you there will be plenty of low stuff in the mix..
mids are centred around 1KHz - this is what cuts you through the mix
too much and you'll sound nasal, not enough and you'll vanish in the mix
high mids - centred around 2.5KHz to 3KHz this is where you get your definition
too much and you get shrill, not enough and the leading edges of your notes will vanish
and it's for this reason that legato centric players [in the Satriani mould] like to roll off this band a little to 'warm up' and 'soften' the tone
whereas full-on riffers and alt-picking shredders like this band quite high to add attack and definition
treble is around the 5KHz area.. this adds all the sparkling highs.. too much becomes piercing.. too little takes the life out of the tone
presence ['air'] is from 7KHz or 8KHz and up.. guitars don't do a great deal up there.. some effects though will generate harmonics up there..
in the studio you'd use this on the mix overall or on cymbals / synths etc..
depending on the fx you have running in the chain you may want to mess with this to see if it adds or fixes something...
but I'd imagine that the impact to the tone overall would not be as strong as the treble bands...