I have not actually played the Hagen, but I have heard the clips and demos: My ears are telling me that the Hagen is more aggressively voiced than the Einstein...Which I have spent considerable time playing. In regards to a comparison to the Herbert, I do own one, and I would say that it really depends on how you define "Heavy/Brutal". The Herbert has an absolutely massive low-end and the mid-scoop feature allows the amp to sound very "sinister", or "brutal". The Hagen seems to get most its "size" from its low-mids and also seems slightly tighter and dryer than the Herbert.
I may be way off base here (again, I have not played the Hagen) but if I were a Diezel sales rep I would ask the question "what other amps to you consider to be brutal/aggressive in nature?"
If you were to answer "Mesa Triple Rectifer or Bogner Uberschall", Tnen I would recommend trying the Diezel Herbert.
If you were to answer "Marshall JVM, Mesa Mark V or Peavey 5150 II", Then I might recommend trying the Diezel Hagen.
One other thought: The VH4 (I also own), I actually do not consider to be a "brutally" voiced amplifier by nature. It is dry and compressed and more appropriate for modern hard rock and many other styles of playing (some VH4 owners will disagree with me). If fact, I have a hard time comparing the VH4 to any other production model amplifier...Simply because it is so amazingly unique in overall character. Also, I find that at least 50% of players that are after a "brutal" tone favor EMG, or other actice humbucking pickups. In my opinion, the VH4 does not mix well with them at all. Where as the Herbert seems to love EMG's.