I would like to ask how is with flexibility at this racquet. I was also looked to models 8000 and 9000 but I think that these racquet are more stiff than 6000 model is which means that is hard to play if you don't have strong arm. For advanced player is 6000 enought or not? I prefer mid flex racquet and models 8000 and 9000 are extra stiff. And of course there is also model 7000.
The 9000S is rated stiff and 9000X is extra stiff. Unless you try them out, you would not be be completely sure which one suits you. Also, the stiffness ratings are always what they seem to be in writing or classified as. For example - AT800 DE which is rated stiff. However upon just flexing the racket, it flexes like moderate flex. Another is the MP88, which is rated moderate flex, it flexes more like a flexable.
If you are an advanced player, there seems little reason to use an NS6000. I'm a beginner and I find the NS9000S and AT800DEF easy to use. There are small international female players under 1.6m in height using xtra stiff racquets and developing lots of power through proper technique. I'm sure you as an advanced player knows it's all in the technique and not just brute strength.