Just read up and am only going to say what's already been said is pretty much bang on.
One of the biggest things I've come to realize is one can't truly appreciate what the Hagen is capable of via YouTube clips. The best I've seen that are all encompassing are by Josh Wibaut, maybe search him up. But even those are still leaving a touch to be desired as compared to the "real deal sitting in front eating my meal" kind of experience. There were a couple soundclouds too, I thought they were great considering they were "all" Hagen, below are the links (Engarn kicks ass here
https://soundcloud.com/engarn/diezel-hagen
https://soundcloud.com/engarn/diezel-hagen-test
Here's my take on the Hagen in a nutshell - can you believe Channel 3 is a fucking masterpiece yet I find myself in "heavy enough territory" with Ch.2 - fact. Channel 1 (clean) isn't just clean, it's got hair on it and is seriously beautiful. I'm a heavier player, but have found HOURS pass by with my just being on Channel 1 - go figure. Then Channel 4 - it's 100% complete and useful - not just a noisier more compressed gainier version of Ch.3 I play this amp ALL the time (and that's outside of the VH4 and D-Moll being ready for action any time). Now, tastes are personal - I can't tell someone they're fucked up if they don't like the Hagen - I can't say it's better than the Herbert - I can't say it's anything other than an amp that FOR ME is not just unique because it's a Diezel (offering that rubbery 3D bounce and punch), but it's unique within its own Diezel family lineage as it's not a mix of anything else. It's its own beast.
More open, more raw, bouncy, 3D, and the one thing I will repeat over and above the details described in above posts (thanks guys) is that with proper technique, it can be very percussive hard-hitting and "dry" like a VH4 death-palm-mute on Ch.2 and most certainly Ch.3 The other thing is it does NOT cover up slop. At all. It's so sensitive that a change in plectrum material is immediately noticeable. Those are the 2 characteristics I'll add to what's already been said or what can be brought up via "Search".
Might not be for everyone; but it's definitely for me. And the louder she goes (3 o'clock on channel, 1 o'clock onwards with MV) the more she opens up and just rips...but still holds tight, deep, crushing focus. It "blooms". That's it that's all
Peace
Unkle Mo