I think you'd be better off getting some sort of distortion unit, either rack mounted big amazing tube thingy, or a stomp box. The thing is that a lot of amps just don't have great distortion unless you get distorion by cranking the amp. I have a Boss OD-3 Overdrive pedal, which is great for a rich and full tone, which I often play with the drive very low, just to get a nice full sound with some crunch. Whether you go for something like that, or something more sophisticated, I think you will get better results than adding more eq to a dirty distortion channel. I would imagine that no matter how much you eq a muddy channel, you're not going to get a very nice sound. Just my thoughts, if anyone cares to disagree, feel free.