FWIW, I don't have a KSR, but I do have a MkII Herbert. From what I've heard, the MkIII isn't terribly different, the main difference being able to program the +/- switch on channel 2.
Anyway, while the Herbert is deservedly known for being a great metal amp, it's incredibly versatile, too. Overall, I hear some Hiwatt in its punchiness. It's definitely a European amp, but has its own sound. The clean channel is capable of everything from pristine crystalline cleans to warm, thick jazz tones. Channel 2- is a thing of beauty. Very touch-sensitive. Channel 2+ can get beastly, but can still easily get into vintage 70s and heavier 80s tones. I spend a lot of time on this channel. Channel 3 is thicker and more compressed, which is a great thing for leads. Every channel is capable of tight, crushing low end. Every channel has incredible clarity.
For me, the trick is to not overuse the mid-cut. It's easy to dial in any of the channels to sound good with it or without it. This isn't to say that its difficult to get great sounds by turning up the channel mids, then cutting them with the mid-cut. Lots of tonal shaping options. If you've ever used a Pultec EQ, it's not dissimilar from the boost and cut on the low band to really dial in the low end, although obviously, with the Herbert, it's the mids.
Some people report difficulty getting them to cut in a live mix, but that's never been a problem for me. The Herbert has plenty of upper-mids and highs. But, as I mentioned above, it is easy to get carried away with the mid-cut, which can cause problems in a mix. It's also very responsive to the guitar/pickup being used. The personality of the guitar comes through, no matter how high the gain is set. I find it cuts better with full-bodied low to mid output pickups, but it does sound good with hot pups, too.
And it sounds really good at low levels, too, since the majority of the Herbert's sound is created in the pre-amp, along with a high-headroom, punchy output section. It does sound its best when it's opened up a little, but I don't ever find the need to use an attenuator. It records really well, too.
The thing I love the most about the Herbert is that it always sounds like a Diezel, unapologetically. It's as versatile as any amp I've ever owned or played.
You might also want to take a look at the Boogie Mark Series, too. I've heard great things about their newer JP2C, and I've got a MkIV, MkIII, Studio Pre, Quad Pre, and Triaxis. They are all incredible sounding amps that do a lot more than just metal. They're certainly very versatile, too.
In any case, good luck with your search. There are a lot of KSR owners here, so I'm sure someone who owns both KSR and Diezel will chime in. I really don't think you'd go wrong with either of them!
