I have a D-Moll. I briefly tried a Hagen but wasn't really knowledgeable about Diezels and how to dial one in, so I don't remember much--good or bad--about it. I did play an Einstein at some point and enjoyed it but it did feel very sterile to me. Hard to explain. I have never tried a VH4 or Herbert so can't comment on that.
Not to throw mixed messages, but there's a lot of MetalHeadMike's experiences that conflict with mine. I also run my D-Moll primarily through a K85 equipped 2x12 (Genz Benz G-Flex, so ported).
The cleans on the D-Moll are some of the best cleans I've heard on a high gain amp. It's like the counterpoint to the Fryette Sig:X cleans, which are a little spongier and bloom out. The D-Moll cleans are clear, percussive, "piano-like" as many would say.
The drive channels on the D-Moll are obviously capable of massive amounts of gain. It did take me a while to realize that the best way to dial them in is with the global master volume up above 1:00, and then use the channel volumes to set level. Keep the gain below 12:00 on channel 2 and let your picking hand really determine how much saturation you'll get. Dig in for more, play lightly for less. For a long time I was getting frustrated with the amp because I couldn't find a good classic rock crunch. Everything just sounded too metal on it.
The presence knob is really important on these amps, both in the actual EQ aspect, but also in how percussive and immediate it sounds.
It's still my #3 amp behind my Fryettes because, like MetalHeadMike, I like tight/immediate amps, but the D-Moll is a great complement to those amps, both in terms of the cleans and the dirty tones. With these three, I don't ever really feel any GAS except for an occasional reminisce about my old Trem-O-Verb, but between the Sig:X and D-Moll I feel like they both have elements of the Recto sound available already to where that GAS is only fleeting and mild.