For sand and even mud, Dunlop 773,
For intermediate, Dunlop 756 RR front is the best front tire I've ever used (but hard to find, you may need to special order). The 756 is a good rear tire, but it wears fairly quickly, especially on dry tracks or trails. The Maxxis IT doesn't have quite the grip and predictability of the 756 rear, but it does seem to last better. I haven't tried the Maxxis IT desert, so I can't comment. (I've got 19 and 18 inch rears, and use the 19 for the track, and the 18 incher for trail and desert riding).
the Dunlop 695 that came stock with the bike was a pretty good intermediate to hard tire, but it wore down fairly quickly. Also, I'm not sure one can be found very easily anymore. Similiarly, the Dunlop 490 front was okay on intermediate to hard dirt, but it didn't match the 756 on the softer stuff or a Bridgestone M-23 on the really hard dirt. I'm using a Kenda Carlsbad sticky rear and Bridgestone M23 combo for hard tracks, and I really like it. Both are predictable and offer good hold. The front hates mud or even silty berms, but it sure grips on hard dirt. The Kenda Carlsbad is a good rear tire, but (like most good hard track tires spun by 55+hp), wears fast.
Hope that helps; if anyone has suggestions for a good desert combo, I'd love to hear it.