Make sure you play one first. And there have been at least 4 revisions to the circuit since it was released.
Dark, somewhat dull and compressed.
Dark and Dull is a good description of it. Which isn't necessarily bad, because the same reason it is dark and dull is the same reason everyone loves them. It has an exceptionally clear hi fi distortion that rolls through the low end without being muddy. But since it sits in the mids- and wants to pull to the midlows, it can sound dull compared to the Marshall distortion profile I think most guitarists are used too. The upper mid sound of the Marshalls
is the sound of a guitar solo. If you like that hot rodded shining bluesy tone, or that bright metal solo sound, the Diezel might not be for you. This ain't a Marshall inspired amp.
I have a VHX, which is their evolution of the VH4. Basically a VH4 circuit, with some digital switching to change the tonal shape. I bought it before ever playing one, and I really do love it. It's an amazing amp. I got it because I'm a Tool fan, and I'll be honest there's some things I probably would have liked to have known before I got it.
Adam Jones tone is legendary. Regardless of whether you like him or not, it is very sophisticated and specific. While he's known for using a Marshall and either Dual Rec or a Rivera, the core mojo of his tone is really seen as his VH4's. They get all the attention as the mythical super expensive amp that's potentially the greatest distortion you've ever heard. Because of how expensive they are, and the relatively low amount of them on the market, most people who want one haven't had the chance to hear one live, much less even play one. The problem is if you're a Tool fan like me, and you've also never played one on it's own, it's a lot harder to hear when he's not playing it; or at least when he's relying more on the other amps. Without having played one in person, I assumed it was a lot more of his sound than it was, and so mentally, there was this unlimited potential to the sounds I could get out of it. But if that was true, then he wouldn't be using two other amps now would he?
The Lateralus solo? Love it, it's the solo I couldn't wait to play on a Diezel. Guess what? You're hearing more of the Marshall than the VH4.
The talk-box solo in Jambi? More of the Marshall.
Basically any of the bright and smooth part of his sound is gonna be coming from the Marshall.
If you want one and you're not a Tool fan, I still suggest you go check out the artist that made you want to get one (let's be honest most of us want Diezel's because rockstars play them), and listen to them, and just generally assume that any of the bright parts of their tone probably isn't from the Diezel.
On the other hand, they are played by rockstars for a reason. There's really no better tone for what it does, just alternatives. It's a magnificent amp. But you might play one and find out that one part in your favorite song you thought was the Diezel was actually something different.