What do you want to do with the amp? What general tone profile are you interested in?
There have been so many different Marshalls over the years that there really aren't many commonalities. Since you are specifically looking for master volume amps I will deduce that you want to be able to get significant preamp distortion and do it at manageable volume levels. Here's my own experience with a few Marshalls:
- I gigged an 800 for years and it's a great amp if you just want that one '80's hard rock/metal sound and can use it at fairly high volume. They are not versatile at all and need to be pretty loud to sound good.
- The only 900 I had was a combo. Tons more versatile than the 800, but maybe a little too much sizzle in the higher gain settings, though with a drive pedal in front they are one hell of a good amp, and I disagree with all the people that kneejerk hated them just because of the diode clipping: they were an improvement over the 800 if only because you can cover so much more sonic territory with them.
- I had a JTM-60 for a while and it was arguably the best-sounding Marshall I ever had but they were notorious for overheating and mine ate up power tubes quickly so I got rid of it. The only other Marshall that I had that sounded maybe as good was a Studio 15, but that was basically a practice amp and didn't have enough juice to gig with.
- Never actually owned a 2000, because I had switched to Mesas, but I have always liked them when I have plugged into other peoples. Crazy versatile and not a bad sound to be had from them, but from my (very) limited experience you need to stick with the 100 watters to get a tight bottom end at gigging volumes, however owners of them can probably give you more info on that.
- Have not had any DSL's. Have almost bought one a couple of times but there is an inherent fizziness in the distortion that seems incurable, plus I can't get my head around a Marshall not made in the UK.
- Recently bought a Silver Jubilee (the 25 watt one) and I can tell you it's the real-deal. Hotrodded JCM-800 tone, but more controllable and more versatile. Its definitely worth a look if you are checking out Marshalls at all, and really affordably priced for one made in England. Strange thing is, I am thinking about sending it back because Mesa just came out with the Fillmore 25. I just have a feeling that amp would suit me even better for those medium gain crunch tones.