Finished the first session with the JS10 KRP yesterday. The 1 gram difference in head weight could clearly be felt while swinging and adds a bit of punch in smashes from the backline. On the other hand, it doesn't cost too much of the head speed which still makes it feel like a JS10 to me. Defense was still better than any other racket I have played with before. The heavier head also made dropshots feel more solid and gave me a better feedback about where the racket head is - that's a thing I was missing a bit with the 4U. Overall, the KRP has a more solid feel to it without being significantly stiffer. It's hard to describe, but it feels like it's a JS10 added with a pinch of "yonexy" feel. So for me, it will surely become my go-to for singles and most probably mixed doubles. For fast level doubles I will have to see if the 4U wouldn't be the better choice. The big question: Is the KRP worth the extra money or not? Answer: I am not sure. Hands down, it is a brilliant racket, especially for those who were missing some weight and punch in the 4U. But if you can get hold of a heavy 4U and are happy with it, I don't think you will ever really miss the 3U KRP. Regarding the black No. 1 - they felt exactly the same as the yellow ones. I strung them with 25/26 lbs. this time (last time 25/25 on the 4U) and personally, I preferred the feel of the 25/25. The ping was ringing longer with the square tension too.