I guess I should have been posting about this all along but I let it slide. But for the possible assistance of others I'll try to condense my experience: If you have a 2017/2018 (possibly 2019?) Subie with a Harman/Kardon head end unit you may have a problem and not know it. They have a defect known since early 2017 which Subaru has not made public - essentially they are subject to a freeze/lockup condition where controls do not work. Often the condition will clear itself if the vehicle is switched off and left that way for a while but a percentage of them with the problem will go completely dead and unresponsive. I suffered through it and know now that the trouble may be present in at least 20% (off-the-record information from my service advisor) of these vehicles.
If the problem is caught immediately it can (usually) be fixed by a software replacement at the dealership. If it gets to the point of death then nothing but a replacement will fix it (and according to another Subie service advisor she had one customer who had to wait three months for a replacement!). I got mine replaced in _only_ ten days but I put that down to making a bloody nuisance of myself at the dealership and online with Subaru national support.
So, if you suspect that anything out of the ordinary is going on with your unit GET TO THE DEALERSHIP AND TELL THEM TO LOOK UP TSB #15-211-17R AND TO DO AN IMMEDIATE UPDATE. If you wait you may be at the end of a long queue waiting for a replacement.
Oh, as an aside, if you make yourself a nuisance with Subaru USA they may well try to make the hurt all better by sending you a gift. They sent me, unannounced, an AO 24 soft cooler and a Tumi travel satchel. I'd have been much happier if they had been proactive and told everyone with an affected unit about the problem beforehand so they might have get the easy fix rather than the hard one.