Your question, to the OP, is one that has kept this subforum alive and hostile for years
There are many camps within the Diezel lineage. And many owners of either or are very devoted to their either or. It's not often you'll have a Herbert owner go off about the virtues of anything other than the Herbert. And the same goes for the VH4. Within the family, the VH4 and Herbert create the most divisiveness. They're both reigning kinds at what the do. Which is better? Only you'll know that - but as I've often said, stick me on an island with either and I wouldn't complain.
The Hagen is a relatively new entrant into the Diezel heavies, and one that needs a serious looksee. D-Moll is the latest, and it's awesome, but it was also designed to be a bit more price conscious in its retail; roll into it the feel of the now-discontinued Einstein, offer MIDI control which the Einy didn't have, and offer the mid-cut EQ shaping that the Herbert has. Where it sits sonically in amongst them all, the jury is still out. But it's a great amp - and people love it.
The VH4 is surgically tight, and records incredibly well. Takes stomps like none other. The original Diezel signature tone. Notorious for its Ch.3 - but all its channels are dynamite. Cleans are percussive piano like, everything else is to be expected - very versatile. One of the most common amps found in recording studios worldwide.
The Herbert is a HUGE amp in Asia and other parts of the globe, monstrous 6 valve amp, well known for its mid-cut Ch.2 "brootz". A little looser of a player than the VH4, and has chimy'er cleans. Lots of chitter chatter about this amp not cutting through the mix - I say "learn how to EQ". Mid-cut is not your friend in a live environment but it sounds wicked recorded. Awesome beast of an amp - AND - you can interchange/intermix the power valves in pairs to mix up the sound.
Hagen is a 4 channel chainsaw of an amp. It purrs in the "brootz" department. Takes drop-tunings with ease. It's got the VH4 signature sound with more of an open, gnawing, burbling decay to the notes. It's very aggressive and organic sounding, not hi-fi. I have the Hagen, I've had many different VH4 and VH4Ss, and experience with the Herbert. The Hagen is a keeper - I'm now back to either getting the VH4 or the Herbert, but it'll likely be the VH4. The Hagen is outrageous. It needs some serious consideration. Great cleans, great raunchy crunch, SICK Ch.3, and a very hot, searing, toneful Ch.4. Awesome amp.
Hope this helps.
Uncle Mo
Edited for spelling