Hi Christian,
That's all copied. There's not much left to try really. You mentioned Aux Cable - is it an older iPhone Model at all? I haven't used an Aux for years since they deleted it off the iPhone? That may be an issue if so with the older bluetooth?
With regards to the Bose Support Website, I am having the same issues and I now worry about the closure of this Community Site - for specialist use like the Aviation Headsets, I think it's a very useful tool in helping resolve the detailed technical issues people have. I help many people here in the UK on a daily basis through my business.
Like you, I tried to generate a return for an unserviceable A20 over the weekend, but couldn't get out of the endless chat/email loop. There's something wrong with it - (I consider myself a 'Power User' too.) I persisted on the phone and held on for ages, getting cut off many times. I am in the UK but was finally connected with their phone support in Manilla, which I thought bizarre, as the service Centre is in Belgium or USA. They instantly generated a standard return for me online, so the need to phone and waste all that time wasn't necessary.
One last thing which may help. If you plan to stick with the A20, I would try another Mic and Downlead Assy? I keep many different ones as spares for the different Aircraft and Heli's whose Customers I support. It might seem extravagant, but as there is a 5 year warranty on the Headset, It sometimes enables you to be able to send your suspected U/S Unit back, without having to be without your Headset? The Mic and Downlead are the most vulnerable parts of the Headset in it's life I find?
I wish you good luck with yours and all the best,
Larry