I ran a JCM900 for years and it was all sorts of brutal. Ran an SD-1 into the clean and blew the paint off the walls. When we played out, other guitar players would ask to use my set up - mainly cause we were all poor and I was the only one with a fancy amp... You could go through a dozen pedals to experiment and get what you want for way less hassle than swapping amps. That Marshall will get you there. It's mildly annoying that there are 2203 different distortion and OD pedals out there to choose from these days, but everyone here can give you a top 10 list to pair with the JCM800 to save you a lot of headaches.
Other things to consider also: speakers, cab and pickups you're using. A great amp and pedal combo into crappy speakers will sound crappy.