For cars with 02xx software there's a new software update available: 0213. It should restore subwoofer control and the settings are no longer periodically reset. It has only been available for about a month now in Holland and Belgium, should be available in the UK now too. I'd really advise your dealer to check for any MMI updates.
(Update won't be distributed via the MMI online update.)
Great news. Thanks for the update. If anyone has a copy of a successful work order for this update/repair, I would appreciate a copy to get things going here in the US.