I just did a quick Google for Understanding EQ and the first thing to pop out was this http://www.soundonsound.com/sos/jul01/articles/equalisers1.asp which should give you somewhere to start.

Sound on Sound is a good magazine aimed at recording musicians in the main, and these guys talk a lot of sense usually.  They also have a forum on their Web site here www.soundonsound.com

Re-posting your question in the Line 6 Lounge as already suggested should attract plenty of attention I would think

Nick

Great question.  Let me preface everything I say by: I'm no expert.  My 10 year experience has only included: Zoom GFX-8 (2000?), Fender Twin red knob, Johnson JM60 modelling amp, Vox Tonelab SE (2009).

Everything just keeps getting better.

EQ is one of those mystical things that no one has a really good write up of but one of the best things to do is just MOVE THE KNOBS.

Eventually your brain can map out what does what, obviously the pros are way better at it too.

In terms of a clinical explanation, I don't know if there IS one, playing with the EQ is all dependent on the SOUND... since it's always different depending on speaker set up, room etc... it's difficult to nail down.  The other problem is, there doesn't seem to be any good examples of "GOOD EQ".  I wish there was something out there that said, HERE IS A GOOD SOUND, TRY TO GET THIS.  That would make it so much more helpful... right now it's like trying to hit a target you can't see!  What the heck is good sound?

RE your question about the highs: I read somewhere that as the volume gets louder, those treble frequencies start to really hurt your ears.  So if you know you're going to be cranking it, you need to roll of those highs.  Ice picks in ears.  Just the way it is, different settings for different volumes.

You can look up a lot of those terms online... But a lot of them refer to the "shape" of the EQ curve...

For example, check out this site: http://mobile.jlaudio.com/support_pages.php?page_id=144

It really helps with graphs, but graphs don't necessarily translate to sound, so the best way is to just sit there and fiddle the knobs because these words won't translate to sound for you

Even though I have general idea of how to manipulate the sound curves graphically... without having a "sound" to aim for, it's challenging...

I'm sure Line 6 means well, probably trying to make it easier and less intimidating for folks who have no idea what a Hz is. But guess what, for those of us who do, it's kinda insulting. I can only speak for myself, but I feel like I'm being called stupid when I look at frequencies represented as percents. And what's worse, all the years I've spent learning how to dial good tones by memorizing what certain frequencies and other parameters related to audio signals do are worthless with the HD500. BTW "Q" is a filter quality factor and as such has no unit of measure, it's just a number that is usually adjustable through ranges like from 0.1 to 2.0 for instance. If Line 6 had gone with standard notation, those of us who know how a parametric bandpass filter sounds with a Q of 0.8 would have it a lot easier.

My opinion is, people who aren't tech-savvy or hate programming their units will stick to factory and downloaded patches anyway. People who already know this stuff have to play it by ear which isn't so bad but it's more time consuming than when you have a rough idea of where to start. People who want to learn their way around programming MFX units will learn how to program their POD HD, and whatever they learn will be useless for programming patches on other gear... or is this what Line 6 had in mind to begin with?