Well, this is very irritating, and I feel for you.
My next move would be to find a nearby In Car hifi specialist and ask them to do some troubleshooting/investigation.
Did anyone else on the Briskoda forum have one of these? There must be others that can do a comparison.
Also, (and not related to the subject), I’m a similar ‘build’ to you (6’6″ and about 15stone) so just wondered how you’re finding the sports seats? You’ve not said they’re uncomfortable, so I guess you like them?