Hailstorm350 has got the right train of thought going...
What you guys are missing is a simple thing that's easy to overlook.
I play bass as well, so I'll talk about this first. My low B string pumps out at something like 30 hz, which is ridiculously low and bordering on in-audible for the human ear. However, my bass cab does not even have a frequency range for 30hz; it only goes to about 50 or so.
Why is this? Because when I hit the low B on my bass, the fundamental frequency is the first to go in the harmonic makeup of the note, so the 30hz goes out the window. What you actually hear is hundreds of harmonic nodes and antinodes etc. that together comprise themselves to sound like a low B. A large portion of the nodes you hear are very low frequencies because it is a bass (the nodes of that low B are naturally going to have many low frequencies).
Now apply this to guitar and the Mesa EQ. The guitar on its own produces lots of low mids, mids, and highs (depending on the fret and string). An E string on its own is something around 120 hz, or that area...This is a frequency dominated by bass, but why is it found on a guitar...? Well, that's just the way it is. But side by side, the bass hitting that same note and frequency will chew up the guitar in the lows due to all of the nodes that follow that 120. The guitar will never truly spit out the 120, because it's a fundamental. The guitar will instead spit out a plethora of nodes located in the mids. Thus, you have a naturally occurring band mix.
When you slide up the 80hz on the EQ, yes you are introducing a frequency that only exists in the guitar as an afterthought in the content of its nodes, but you have now emphasized it so that the nodes of the 80hz are emphasized even more than previously. These may be, for example, 90hz, 95, etc (plus many many others). Even though you have boosted this frequency, the bass is still pumping out its 80hz stronger than you are if the volumes are set correctly. Heaven forbid that the bass player should choose to boost something around 30 hz and some low mids...The result will be total ownership of that frequency range and the only frequencies you will hear coming out of the guitar are mostly mids.
I hope I said all this properly, if not someone please correct me. It's been over a year since I looked at musical physics (and I'm not physics guy!).