Hey,
I don't have any experience with the newer YZ/WR's. However, you are correct, they rev to the moon and the mid and top end is where they shine. They do have low end grunt, but I don't think the motors are happy like that, like the XR's are.
I share your appreciation for the John Deere stump pulling power of XR's. I've still got my first XR, an '87 XR200, and a '96 XR400, and now I have the '00 XR650R. Just coming off of my 400, I must say, the 650R handles quite a bit better, even in the tighter stuff. The forks/shock on the 650R is better than the standard XR suspension I'm used to. With the aluminum frame, the 650R would probably even feel lighter compared to your 600. I know for me, the 650R doesn't feel any heavier riding it compared to my 400.
The power of the 650R is true-blue XR power, with all the bottom grunt your arms can take, but with a real nice midrange pull, and it even seems like it revs a tad higher than my 200 or 400 ever would. On the occasional road section, the 650R will pull 70 almost all day it feels like, with a supposed top speed of ~100mph with the stock gearing (I believe it, but havent' tested it).
If you want a different bike, you would probably learn to enjoy the WR, just as I would. However, take a look at the 650R's, I got my '00 with very low hours for $3200. Good luck.