Couple of things that will hopefully help you to find the answers to your questions.
1) Springs are designed to absorb impact and return to their original shape. In addition to this they also play a large part in ride height and quality. Personally I have never seen a factory spring require replacement due to any of it's factors having worn out over time, unless that vehicle had suffered a great deal over a very long period of time. I have seen a number of cars that have had badly modified stock suspension that have absolutely ruined the handling qualities from stock however. The easiest way to work out whether what you have is wrong or not is to go for a ride in a similar vehicle. Describing vehicle qualities always leaves room for personal interpretation.
2) At 70mph you should feel more drivetrain harmonics than you would at 30mph. My 560 certainly has more feedback than modern vehicles at 70mph but it doesn't really change all the way through to 120mph. A simple wheel balance will ascertain whether or not you have a problem with a wheel or tire, and balancing should be performed reasonably often anyway.
For what it's worth I often think that my 560 makes a lot of squeaks and rattles but my passengers just do not hear the same things I do, over time I become more sensitive to things I guess.