100000% this. Diezels have a wooly muffled thing going on, just as you describe. Works for rock and some slower stuff but it lacks bite and aggression like you can get from other amps. On more aggressive amps it’s easy enough to scale back, but on some amps you can never add it in.
I was going to say that Diezel was my holy grail tone, but it isn't really true. I loved the stuff that Adam Jones has done using a VH4, and I love the Metallica tones that used a VH4, and plenty of other bands like Muse and what not... but my first Diezel was the D-Moll. I think it is great! But it really needs a high-frequency boost in front of it. Either an SD-1, TS808, or an EQ pushing all of those lovely high frequencies into the front of the amp.
The VH4 I bought on a whim. I do like it, but it is quite vintage voiced, especially compared to something like the Dual Recto or Mark V. I've recorded some great sounds from it, but it always seems a bit undergained and old skool sounding.
The Diezel Hagen.... honestly, it isn't really for me. It is owned by the ex-TNBD guitarist, and when he moved out of the UK, it ended up living with me for a few years. I think he wants to sell it, so at some point I'll be doing that for me. It sounds modern, but there's a texture to the distortion that I don't enjoy as much.
So really, I have two Diezel amps. I really like both of them, but they're not my primary amps. The D-Moll is my favourite.
On the last TNBD album I used a Fryette Sig:X and the VH4 together. Two 4x12's and two mics on each of them (421 and 57). They just really went nicely together. Listening to the Mark V though, I reckon if I had had that back then, I could've just used that amp by itself.
Anyway.... I do feel like the Recto and Mark V are more "me" - even more so than the 6505+
I love amps, so not really planning on shifting anything any time soon. You can get some really good deals on 100-watt amps, coz a lot of people don't want them as they're switching to modellers. Which is fine - better tones for me!!