I removed the OEM amp and replaced it with an audison DMI which receives the most input that went to the factory amp.
I think took the toslink output from that to connect optically to am audison 12.14 bit. You could fit am 8.9 bit if you are prepared to tie some channels together, eg. the centre channel could be fed from the left and right, and the rear deck speakers could be fed from the rear woofers. This will bring you from 9 to 6 channels. In my case I amped them all.
I bridged the spare channels in the audison to drive the underseat woofers with more power. These absolutely do not need to be upgraded in anyway, the stock speakers are much more than adequate to sound like a proper after market boxed sub in the boot.
I bought a loom adapter from ebay:
https://www.ebay.co.uk/itm/255748549...edia=WHATS_APP
It's the match product - it was easier because it was pre labelled but there are cheaper options. This was cut this up and joined to the supplied loom with the 12.14 forza amp in order to create a PNP loom adapter.
After that, it was just configuring the audison amp to do the right stuff and then tuning.
The amp and DMI fit in the stock location under the pull out tray on the left hand side of the trunk. Nothing is invisible at all, and it runs cooler than stock as it's a class D amp.
All the stock tones, and phone etc work as expected, with no echo on the bluetooth connection when making calls.
I use a MR12V for my sound source playing just carplay - can't remember if there is a high res subscription - I think not.
Half the battle is the set up which I did not do.
I wish you guys were in the UK to hear it. I kind of fell into doing this, not being sure how good just an amp upgrade would be.
Well worth it.
Other upside is the only thing proprietary to the E92 is the loom adapter I made. I could take it all out, swap the stock amp back in, and use the DMI and amp in many other cars. Some newer models will use a different option instead of the DMI to convert to the digital input of them amp, but it's still usable in many BMWs Porsches, audis etc that use a most loop for their audio. Some newer cars use a different most lop that operates at a higher frequency.
Only negative I can think of is the oem equaliser built into the CIC no longer operates the equaliser as it was inside the amp not the outgoing signal from the head unit. I don't find this a problem as everything as bass and treble still work and are set flat. The MR12V has a more granular equaliser if I wanted to use it, but that too is set flat and the car tuned to my ears.
I can't imagine any reason to adjust it - it's just perfect.