Ditto for the Miele.We have just got a new bed its legs are a smaller footprint and in a different position to the old one which had been there for a very long time, as @raleighnut says above a good vacuum on full power, I removed the extensions and just used the round outlet going over and over until it looked reasonable, I can still see where its been, but it is a lot better.
Miele Vacuum used, other makes are available (but not as good)
Steam and heat as above but not wire brushing, maybe a tooth brush. If your underlay is foam you'll have more success than with felt. It will improve with repeated attention.
What about turning it over ?