B&O upgrade in B8.5

I’m pretty sure it’s a major wiring change as the amplifier is in the boot and a concert car is not wired for it.

On a slightly different point, I wouldn’t bother. If your putting the work (and money) in to retrofitting something you will get defat better results from some good component speakers and a DSP (with built in amp). There are loads out there, the Audison AP8.9 bit is a fantastic bit of kit that has a 9ch DSP as well as having a built in 8ch amplifier (tweeters and components front and rear); the remaining DSP channel is used to supply the signal to a subwoofer amp.

The B&O is and upgrade over the standard, but it’s not great and not worth the money in my opinion, either to retrofit or as a spec option from factory. Both the Audi Bose and B&O systems have ok mods, no highs and no lows. Spend the money on a proper aftermarket system and you will get a full dynamic range and a much better sound stage.

Agree with @OliNix Spend your money on aftermarket. The B&O speakers will be better Than the Audi Sound System, or whatever level the car was fitted with. But certainly not for the cost and ball ache of sourcing the equipment. A Digital Signal Processor, high end component speakers for the front, 10 or 12 inch sub and you'll have a far better sounding system than the B&O.

On top of that, it's highly likely that the speakers are not even B&O originals, rather a licensing agreement with a third part (I think it's ASK in Poland, bit I could be wrong).

I had the Concert in mine, non mmi, standard 10 package speakers. Upgraded to all aftermarket and very happy I did, particularly when I see other A4s with stock looking/sounding systems

Phone around to you local car audio specialist. I'm sure they'll put a package together that would sound good and include fitting.

Where are you based?

Also, if you're looking to upgrade functionality, forget adding DAB modules etc. Go straight for an Apple CarPlay/Andriod Auto interface retrofit. This thread tells you everything you need to know.