To answer a couple of comments/questions, I just simply tightened all existing driver mount screws to a secure/snug level. I did not try to apply a lot of force or torque but tried to bring all the screws to about the same point of rotation. I did this with seven speakers associated with two different systems. As I mentioned before, some screws were already very tight, some I could move maybe a quarter turn and some were loose enough to require a couple of full rotations. The benefits to the two channel systems were readily observable but not earth shattering by any means. The speakers in the HT room were harder to get a gauge on, but overall I would say there was at least some minor improvement there as well.
It's been interesting to see some of the rather skeptical reactions to this post. All I can say to this is that tweaking a system is a process of trial and error, but the cumulative results of staying open minded and incorporating minor changes can have (and often does have) major improvements to a system. That can be true even if the system is performing already at a very high level.
Tweaks are typically the least expensive things to do to a system. In this case tightening the screws took all of ten minutes or so. I then had the pleasure of listening to familiar source material and finding a trace more detail present than I thought possible to extract.