Well.....I have my opinions and will make this statement and then break things down a bit. All Diezel amps can do every type of thing. The Einstein is the most organic with the most overtones. The Herbert is the most saturated but not by a tremendous amount and I feel the Herbert it the most forgiving as far as lead playing is concerned. The VH4 is the fastest amp, Heavy, CLEAR, ARTICULATE. People "wrongly" or "mistakenly" say the VH4 doesn't have as much gain as needed for metal. THIS IS FALSE. Gain and saturation are not the same thing. Because VH4 is not as saturated, it gets categorized as a rock amp. Let me remind you that Hetfield plays a VH4 and Sambora a Herbert. The VH4 has a very pummeling immediate punch to it. All of the Diezel amps have their own thing going on. The "Diezel sound" may take a bit to get used to for your ears.....it is time well spent. All the amps have fantastic clean channels. All the amps have fantastic lead channels. All the amps can chug nicely. In the chug department, I would say the VH4 is the tightest, then Herbert, then Einstein. The Herbert is and does sound mean due to the wattage yes but also the mid cut features. All of the amps sound good no matter how they are set (within reason of course).
Einstein - might be my favorite of the three. It has a sweet aspect to it that is so full, smooth, and clear.....dare I say romantic? I would say romantic. It gets very mean, has a singing quality about it yet can seduce you with its brilliance. It is an incredibly fun amp to play (as are all Diezel Amps) but there is something special about the organic, singing, sustaining aspect of the Einstein. Ballsy gain in spades and cleans to die for. Mid tones that sing and hit you right where it hurts. The Einstein IMO is FANTASTIC! Doesn't get the respect it deserves for metal but if you need som high gain, cleans and play music to where chords can ring out and sustain, this amp is very very good for that. It takes a pedal very well if needed. I myself am not a fan of pedals but the Diezel amps take them very well.
Herbert - Said to be the metal monster and I can not disagree but would you think I was wrong if I said the Herbert cleans are God like???? They are that good. Seriously. I think the herbert excels at the metal tones and the clean tones the best. Yes it can do the in between wonderfully as well but for that I may tip my hat to the Einstein or VH4. The Herbert sounds fantastic at low volumes as well.....all of the amps sound decent at low volume but really shine when pushed...(that can be said of any tube amp of course). The Herbert is forgiving from a lead perspective. You can pull things off with that amp that you can't with other amps. It is a very fun amp to play indeed. You don't have to fight the Herbert. It gives you back what you put into it and then some which is very cool. The feel of the Herbert is nice and responsive. It is tight with great attack and just enough sag to not make it stiff. It sings, it is heavy as hell yet immensely beautiful on the cleans. I would say it rivals some noted "clean amps" as far as clean tone quality. It records wonderfully. I will say some falsely have said it gets lost in the mix.....I say falsely because any amp will get lost if you dime the gain and bass and cut out the mids. The amp does well in the mix if used as it should be.
VH4 - This amp is a blast to play. The overall features alone make it awesome. The versatility of the VH4 is hard to match. It is said to be stiff but I don't find it to be. I do find it a down strokers dream if you actually work and play it. The harder you work the VH4 the more it gives you. It is full of tone, gain, and will hit you like a ton of bricks. It is so clear and fast that you think the amp may not be "heavy" but if you think that, you have deceived yourself...lol. The VH4 is very bad ass IMO. You can coax great cleans, lead and riffage possibilities out of it. It is immensely responsive and records and cuts great. I feel the VH4 is the least forgiving of the three amps mentioned here. It's lack of over the top saturation is mistaken for not enough gain but it has gain in spades. It really can do anything well. It is the fastest amp IMO. It is the most responsive IMO and it is anything but dry or sterile. It will expose your sloppy playing but it will also make you a better player.....actually any of the three will. Push this amp and it will greatly reward you with wonderful sounds and responsiveness. It has great low end, as does the Herbert. The VH4 low end is still clear and very full....again it is deceptive. You hear such force but yet everything is clear and articulate....not muddy.
This said, a good cab will do wonders for these amps. The right cabinet seems to make these amps shine even more. The build quality is second to none. The customer service will never be compromised. I have had all three for a while and each time I plug into any of them I still smile and find something fresh in them. Diezel amps are incredibly musical amps.
If this is not enough information, please feel free to ask me further questions and I will do my best to help you.
I will give you this word of warning....if you like one you will end up with all three...lol. Seriously. You just wont be able to not get them all..
Ahhhhh......and the new cabs we have coming out are flat out AMAZING. You will be blown away I promise you.
you'll like both 3 of them, Herbert, Vh4-S, Einstein 
