One thing I have noticed with Diezels is that I prefer them biased cold. I'll admit, I first tried it because of Adam Jones, but damn, that guy is right. When you lower the bias from 60%-70% to 50%, the amp just opens up and becomes a whole lot clearer. All of that congested midrange that can be in Diezels sometimes just melts away. The amp becomes sharper, clearer, and more open. It'll accommodate higher output pickups better, too.
If you have an AxeFX you can try this on the Diezel VH4 (Blueface or Silverface, doesn't matter). Go into the Amp block, to the power section and lower the bias to 50% (or even a little less if you want). You'll lose a smidge of gain but all the congested low mids clear up. Give it a shot!