The tube screamer actually pushes the gain signal on the amp and gives it more gain stages, slightly more saturation and tightens up flabby bottom end. What most people do is possibly put gain slightly more than 0, Level at 9 or 10 and tone at 5 or 6. That's enough for a nice clean boost and to tighten the tone up.
The JCM900 should sound metal enough on it's own. It's supposed to have more gain than the 800, especially the master volume models. Just make sure he's not scooping his mids, perhaps change tubes. They make a HUGE difference to sound.