The Herbert markets itself as an "ultimate high gain metal" amp and does a good job selling that idea. The super high tech looking logo, the cool black metal faceplate, the blue LEDs, the mid scoop, three channels, etc.
My opinion is that Diezel amps just aren't very good for modern aggressive high gain tones. Basically, the visual design of the Herbert writes checks its circuit can't cash. That goes doubly so for the VH4. Diezels lop off a ton of top end articulation at the amp's input and there's really nothing you can do to negate that or bring them back, not even with a treble boost or OD pedal because all those added highs will just get lopped out of the circuit, and as a result the amp has these weird, overwhelming splatty mids that destroy any ability to make the amp tight, articulate, and as aggressive as you might expect it to be able to get.
Everybody always counters this general "Diezels = bad for metal" argument with "but James Hetfield and Adam Jones" etc. but that doesn't really work. Those guys run multi-amp rigs. James uses Mesas for tightness and treble detail and the VH4 for low end and mid thickness (doesn't work if you ask me, James' tone has been splatty, woofy, and awful ever since he added the Diezel and then after that when he went to modeling and now uses a blend of Diezel and Mesa amp models. James' tone doesn't suck now because "modeling" it sucks now because he's leaning more and more on the Diezel sound whether by real tube amp OR amp model) and the same goes for Adam Jones. The tightness and aggression in his rig is all in the Marshalls or Mesas.
Sorry man, if you want aggressive tight articulate heavy metal tones, Diezel amps are just not the answer. I know it sucks hearing that when you have one on the way but hopefully it helps you get where you want to go faster in the long run.