Hi Lars,
hard to say, how you should handle that.
For how long have you had it?
What are the reasons, why he won’t take it back?
Is it an official B&O dealer?
in the end maybe only a lawyer can help you.
There might be some settings in the Netflix app, that you have overseen.
Netflix nowadays use Dolby Digital Plus, which the Avant can handle.
If you use media (from bluray dics or ‘ripped’) with the socalled HD sound formats, these must be decoded in the playback device (e.g. a bluray player) - this can not be done by the tv.
Dolby ProLogic is hardly used anymore - is a relic of the analog days.
The MK1 platform/user interface was made in Struer (the last one) and had some added Smarttv functions (Webmedia), which nowadays are pretty obsolete.
However they do not disturb anything there - most people use an AppleTV4 or a Chromecast device for the SmartTV functions on the MK1 anyway.
It is generally seen more stable than the MK2 - and most people like the interface better.
However, you must know that it is older.....and no tv lasts forever!
Most people avoid the MK2 - which was kind of an inbetween model - and go right to the BV Eclipse (which is pretty stable) or stick with the Avant MK1.
A friend of mine however, has an MK2/75” and is happy with it.
So there is no 100% pros and cons here!
I guess, that if your MK2 did not have this fault, you might be happy with it too.
Good luch with arguing with the dealer - and let us hear how you get along.
MM