I have the 9833 in my car (I guess one notch up from the 9831?), it replaced a 9813 which is now in my boat. It is pretty good, but I think that I like the 9813 better. The 9813 is basically an older version of the 9833. They have the same features on paper, but the 9813 seems to access faster, especially MP3/WMA disks. I also feel like the 9813 sounds a little better??
Overall I have been pretty happy with both, but I personally feel tha Alpine has been going downhill for a while (especially looking at the specs of their recent offerings on Crutchfield).