I figured I could give a fairly accurate view of the JCM 800 since I own an old 1971 100 Watt Marshall Super lead and I've borrowed a 1979 Master volume Marshall 50 watter for close to one year. 

It's important to note that this particular amp had been sitting for a very long time.  I'm not sure what sort of weather it experienced, but odds are that it was not ideal.  This tells me that the likelihood of capacitors sagging is much greater.  In other words, the tone circuits in this amp may be different from their intended values.

At first, Heath played on the JCM 800 through an Agile SG.  It's important to know that Heath's guitar style is very laid back.  He comes from a classic rock / country type of background and his guitar playing sounds like good old back porch type of playing. 

When he played the tone was very creamy, even with the bridge pickup.  The amp was thick and far from aggressive.  The amp was very dark no matter how I set the amp up. 

Just for the hell of it, I put my THD Hotplate in between the speakers and the amp.  After cranking the amp up, the tone didn't drastically change.  It still had the same vibe.  It was very dark, but very think.  It was still very mellow.

I asked Heath if I could play.  It's important to note that my playing style is the complete opposite of Heath's.  I play like a bat out of hell.  While I don't listen to it much anymore, my playing is straight out speedmetal with your hair on fire.  I spent days on end getting as much aggression out of a guitar as I could.  Now I consider myself to be a pop music guy.  Strange.

Anyway, when I picked up the guitar, the amp seamed to kick it in gear.  The amp seam to react amazingly to the style I was playing.  In other words, if you play mellow, it sounds mellow.  If you play mega aggressive, it sounds mega aggressive.  Very cool.  When going for all out death, the tone reminded me of Ride The Lightning.  A very cool sound.  A very big, aggressive sound, but certainly not a Boogie Rectifier. 

Somewhere in the review, I did my old trick where I put a yellow Boss overdrive pedal in front of the amp with the gain on the pedal all the way down.  It worked great with the JCM 800.  I know Zakk Wylde used to do the same thing back in the day.

I must admit that the tone wasn't overly different from my other Marshall amps.  It seamed to have a thicker low end on the 800 and not as much in the high end.  Ironically, after playing on the amp, the first thing I said was "I want one".  So it must have been different enough.