hey, I heard that...
You should have dithered the subs Mike
Seriously, yes, as Peter mentions subtle mottling is present, but it does not distract from the beauty of the overall scene. Its rather "ballsy" to upload a 1:1 image in the first place. Most simply resize/downscale to hide the problems, much like plasterboard really. Really, a top shelf piece of work. Thanks for the opportunity to have swim.