Tyres I've used on my 640 and 690: Full offroad, Dunlop Geomax AT81, on the aggresive side Michelin Desert, Pireli Scorpion XC. More dirt touring orientated, Mitas E09, Michelin T63, Dunlop D606. Less offroad, Mitas E07, Metzeler Sahara.
Deserts are great but they are pricey. Actually, I like all the tyres on the list which lean to offroad use but I am a cheapskate so my combo of choice is AT81 front and T63 rear. A set will cost you around R1500