To me, there are few basic types of valve amps. I come from a metal background so that affects the way I look at the following...
1. Marshall
The most known and one of the most popular amp makers ever. Marshall has a really organic grit. It really grinds and you can adjust it anyway you like, it'll always have that signature marshall tone (which some prefer, some don't). The tone itself is a bit tight, like perfect for hard rock-type of stuff but not in it's comfort zone in heavier music. Warm cleans that get the job done, nothing earth-shattering. I think most still buy Marshalls for the name and logo, dunno, to me they have never really shined but then again, I don't play that much classic rock or that kind of stuff. There are of course other amp makers that also fall into the marshall-tone category, from what I hear Splawn is one of the more popular and promising ones.
On a curious note, Nile has recorded some wicked tones on DSL100s but they have this whole different sound world going...
2. Boogie
Here we have two types of tones actually. The Mark and the Recto. These are two different beasts altogether. The Mark is a bit more like a Marshall but it takes the organic feel a leap further and while the Marshall can grind, it feels more like a soft sizzle compared to Mark series. Mark series allow you to maintain a really responsive and organic tone but still take it to the extremes. The result is a seriously tight metal crunch that has a really meaty and aggressive tone.
The Recto of course is one of the most easily indentifiable. A tight sizzle in the top end and a larger-than-earth bass thump that you can feel in the chest. No wonder Mesa has a huge array of amps in the rectifier series. They are a bit of a tricky beast to tame since so much bass can easily turn flabby without some precautions. A booster pedal is recommended here to cut the bass from the signal coming from the pickups and to compress it a little to make Recto deliver a bone-crushing wall of sound. Rammstein would be a good example what the recto can do.
Diezel I believe falls a bit into this category, like a hotrodded recto that costs an arm and a leg.
3. German robots
This is a debatable category, but let me share my insight.
ENGL, Bogner (Uberschall mostly) etc. What they have going is a sizzly and tight bass thump, minus the character and organic tone. Now this isn't necessarily a bad thing. If I needed a tone that is so balls to the wall that it's ridiculous, ENGL would be one of my top choices. It has a tight metal crunch written all over it. I myself prefer the mesa mark series but ENGL can take a sizzly bass thump -esque tone and take it beyond, it won't have the same organic and woody tone that comes from the guitar as in mesa and marshall toned amps, but you can get a grind happening that'll pulverize small animals. The clean is *very clean* rightly according to the german engineering -spirit, it's almost too clean, massive headroom here. Overall the German robots are very cold and not digital but a bit solid stateish at least that still have the powerful tone of valves.
There of course are exceptions as in everything. I've been hearing a lot of great things about the Invader for example having a lot of character and tonal capability.
4. The face(plate)less ones
These are amps from mostly boutique amp manufacturers and some of the less popular but big companies. It's the amp type that you just can't get your head around. It's *ok* and *nice* here and there but there's no wow-factor or anything that jumps out at you. It'll probably get the job done and with careful tweaking and mic positioning you can record some nice tones but these are not the tools for making your signature tone. Now I can't go naming that many amps extensively since I've only had my fair share of amps to play with but Hughes & Kettner are one that have never appealed to me in any special way.
There are a lot of unique tones in the amp world but most of the amps' voicings can be categorized like this. While it's good to hunt for an amp that gets the magic happening for you it's also good to know what ballpark you should be looking in. Hope this helps.
ps. 5. Koch = fuzz pedal that looks like an amp
