D606 is a great front tire for the 990 and handles on the highway like a regular tire. I get about 4000 miles and then the highway behavior gets poor due to cupping and squaring of the tire profile. Still does well in the dirt like that.
Since I have some more highway touring coming up I have changed out to Mefo's until next years riding season.
I think the D606 is rated for the 990.
KTM 400 XCRW with Thumper Racing 450 kit and 450 cam 2007
KTM 450 XCRW with Christini AWD and Plated 2008 now has 520 Thumper Racing kit
KTM 990 Adventure 2008