I realize that this is an OLDER thread, but I do in fact have relevant information to add!
It's unfortunate that the OPnever came back to say what happened w/ his car, as I am 100% sure I am facing the same issue currently with a 1992 600SEL I am repairing at the moment. I have replaced the rear accumulators, rear struts, and Steering angle sensor (all NEW Genuine MB parts), and while there is NO ADS warning lamp on the cluster, the system is still stuck in FIRM. I can force it to go to Comfort in HHTWIN (STAR/SDS same thing), and it works just fine. I have also tried replacing the ADS computer (used) and it didn't help.
Another thing I want to add, is that I have watched the ADS actual value readings on SDS, and noticed that the system actually DOES energize the valves at engine startup, but only for an instant, and then goes RIGHT back to FIRM. Hopefully we can figure out this dumb, goofy problem!