Knox seems like a good company dedicated to producing quality goods. Again, webbikeworld has several reviews of their products. Reading those articles gives an insight into what to look for in back protectors. I am skeptical of Dainese and Alpinestar protectors as I do not believe these companies make these products. They are simply attaching their brand to someone elses product. Who knows what criteria they used to pick them. I’m sure quality was part of it but I’m also sure dollars was a big part. You may be paying premium dollars for average goods with a brand label.
The bottom line is any of these products are likely to be superior to the protectors shipped with jackets. The elbow/shoulder pads are often CE certified but the back protector almost never is.