That's probably your problem there, if you have your mids and highs cranked like that it's not gonna rock like you're wanting. It might be good for lead work but it's gonna kill your backing. try something like bass:7 Mids:4 treble:4-6. I'm not sure if that's what you're looking for, but I like it.

I just thought of another couple things-

With a lot of tube setups you can make your sound more distorted (in a good way) by turning the input volume way up and lowering the output volume.

On some amplification/distortion equipment the bass/mids/treble knobs work by feeding that part of your guitar through the system at different volumes. What you can do with this is turn all of them down so the sound that's output isn't as much your guitar as the distortion. You gotta be careful you don't lose definition when you use this, it's really easy to turn your sound to mud like this. I don't know if this would work with your setup, it's something I've seen more on things like 20 watt practice amps than on stacks, but it might be worth checking out.