Sounds like you need a Marshall Powerbreak - it allows you to crank it to 10 and get the tubes burning at their full potential but you can control the volume externaly and bring it back to what ever level you want (bedroom level too).
If you are looking for info on tube replacement I recommend Bob at eurotubes.com He did my Mesa Boogie Triaxis and Mesa Simul Class 2:90. Just tell him the sound you want and he will spec it up. If it's a new amp it shouldn't need new tubes though...
Like Kevin said though, the pickups on your guitar and the room accoustics have a lot to do with the tone. Never used a JCM800 but the 900 has animal distortion - very powerfull!