I have a 990 and got it new. I don't think I would trade it for anything other then a 990. Cut is great, doesn't take alot of hp. I had a RM60 before and it took more hp then the 990 does. For a wide mower I have never hit the ground with it, but I don't mow ditches and mow about 4" high. I mow around 20A with it and it's just right. I have pulled it with a JD950, Kubota 3010, Kubota B7800, the little b7800 has hp but going up a hill it would pull it side ways. I pull it with a kubota GL4240 now and it does a nice job. One thing that I make sure I pull the blades every year even if they are ok, because the pins that hold them well rust up real bad and makes it real, real hard to get them out.
The model RD990X that you are asking about, I guessing the X is just the lates Model. Mine does have a letter after the 990, but forgot what it is. My the way get the chain kits front and back. I added 2 links to mine and at 4" they just touch the ground, just don't make them to longer that the blades well hit them.
By looking at my post you can see I like my 990. You might want to look at there bat wing, the one that's made up using 3 of the 990's I'm sure your JD can pull one.