I don't think I buy your point, since the only two servo sub examples I'm familiar with, the Velodyne DD Plus series and the Rythmiks, both use very high-end custom drivers. I wonder, however, how audible the differences are, or whether they are measurement-only wonders. I can't hear much of a difference in the settings on my DD18 Plus. Many product features offer measurable differences that are very difficult for humans to discern, and many companies make a good business convincing people they can discern the differences. K&N air filters for cars and trucks come to mind are an example that come to mind. (Though K&N filters are not especially good for your engine, while sub servos appear to be innocuous.)