Remember also that if you need to be a little more aggressive than you would like with your NR, consider treating color more aggressively than luminance noise. In the crop posted earlier, it is apparent in the image that, in a YCbCr system, the Cb and Cr components exhibit large blocky compression artifact. These can be treated aggressively to remove low frequency, typically green and magenta, color blobs and restore more natural color. The higher frequency "grain" is primarily in the Y channel and can be treated less aggressively. The color is way more distracting in this image than the higher frequency Y channel artifact.
Finally, after noise reduction, consider adding back some "film grain" or whatever it may be called in your specific application. This will usually restore a more natural look to your image, breaking up the plasticky look of aggressive noise reduction and potentially adding some acutance (perceived sharpness) back into your image.
Attached is the YCbCr channel representation of the crop. You can see all of the JPEG blocks easily in the Cr and Cb channels.
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