In general:
180-250 Hz will add thumpf, you know, the pump-in-the-chest drive.
400-600 Hz will add a fuller body.
800-850 Hz adds a harder, Tom Scholz / Boston sound.
1120 or so Hz can create the ESP Active EMG old-school snarl tone.
Around 2000 Hz adds a harder, more edgy tone.
All of this is adding something, also known as additive filtering.
The other direction, removing something, is called subtractive filtering, which is often overlooked, because many tends to naturally want to add to their sound.
Lets say, as you've stated, that your amp sound a Bit too scooped in the mids.
Maybe you hear it this way because it really is scooped, but if in reality it's only a small touch scooped, how you perceive it could be because the ranges above and below are really a Bit pumped up.
If so, instead of adding more mids, you might have success with gently scooping-out some of those ranges.
Sometimes, removing something else than the immediate felt problem can result in greater clarity, where adding what immediately seems to be missing can lead to a more swollen tone.
I'm not saying this is what you should do; it's just a suggestion.
But hey, you got a graphics EQ, so go experi
mental, dude
As a comparison, the ADA MP-2 has a fou-band stack and a 9-band graphics EQ.
I used to add all kinds of small little EQ'ing with the EQ, and could fiddle forever, until I learned to keep it fully flat, setup the 4-band first, and then, using the 9-band, lift a few spots just a touch - and cut away what was intruding into the soundscape, i.e. using subtractive filtering.
This is actually the same approach photographers use.
Rather than trying to flood the studio with lights, use a black panel on the other side of the model to remove some light there, in order to create a more felt contrast.
And now to the fun department..
Of course the ultimate solution to your problems would be to have the amp modded with different power transformer and power tubes, and replace those pesky scooping 12" speakers with some harder sounding ones.
Yup, I'm in that mood tonite; it's just meant as a joke - well, not that there isn't some truth in such mods, but it can be a rather expensive solution.