My SDV8 AB has the Meridian Reference system, 1700w and 29 speakers, and I'm keen to understand which is the best input to use so that I get the best sound production from the music on my iPhone.

I'm currently streaming music from iTunes and Amazon Prime via Bluetooth and occasionally using CDs.

The choices as I see it are as follows :

Bluetooth
Aux-in
USB
CD 2019 L405 SDV8 Autobiography
2023 Defender P400e HSE X-dynamic - gone
2020 Defender P300 SE - gone

2019 Jaguar F-Pace SVR - gone
2013 SDV8 Autobiography - gone
2011 L322 TDV8 Vogue SE - gone
2006 L322 3.6 Vogue Se - gone
1999 P38 4.0 se - gone
199? Classic 3.5 - gone

CD will be best.

most sources apart from CD have compression or other factors of various sorts which have a negative impact upon quality

CD definitely

In order of highest Q / bit rate / less compression
CD
FM radio
High bit rate mp3
Dab radio from primary sources
Low bit rate MP3
Dab radio from secondary sources

If you have the surround / meridian effect on ? this will change the pure sound into ( still good ) but less pure due to the phase difference built it to the effect .

An Iphone via bluetooth is unlikely to give the best quality although it may flatter to deceive , it will depend of the source you originally got the music from .

can't speak for the other streaming services, but if you only have the basic Amazon Music, their streaming sample rate is quite low, hence they off their premium"HD music" (at a cost of course)

I plan to use my iphone via the USB input using a quality cable form AudioQuest. My phone has a mixture of MP3 and apple lossless formatted music.

My phone is an iPhone 7 Plus and as such it doesn't have an onboard DAC so the raw music data should be sent directly to the amplifier in the Meridian system for decoding. 2019 L405 SDV8 Autobiography
2023 Defender P400e HSE X-dynamic - gone
2020 Defender P300 SE - gone

2019 Jaguar F-Pace SVR - gone
2013 SDV8 Autobiography - gone
2011 L322 TDV8 Vogue SE - gone
2006 L322 3.6 Vogue Se - gone
1999 P38 4.0 se - gone
199? Classic 3.5 - gone

I'm no phone expert, but how can Ann iPhone NOT have an onboard DAC? It is able to play audio through its own speaker.

Yes all devices that convert digital audio to analog Audio must have a DAC that's what DAC stands for, however there are many different types of chipsets and a high quality source connected via the jack as opposed to Bluetooth as Buz is doing will be the most pure available for that type of device .

Pragmatically, lossless music (AAC?) at a high bit rate (up to 320kbps) via something like Tidal and using the USB connection should be best.

CD? What's that?
Convenience (and tidal) killed CDs for me a long time back.

And, you know what, try different things out (including the various Meridian DSP settings) and just listen to what you like best and not what "should" be best.  L405SC just arrived... still working everything out!

I've been using my iphone via usb to access my iTunes library which has a mix of apple lossless and mp3 files. I also have amazon music hd which also plays through the usb. Recently though I've lost the ability to access my iTunes library through the screen, or indeed skip files through the steering wheel controls. I've tried a reset, restart and repairing but I only ever get a temporary resolution, music plays but I can only control it through the phone itself. My wife also has the same problem in her evoque. Has anyone else had this and is there a solution? We both have iphone 6s phones.

Get a blackberry

As above - Tidal - fabulous! 2017 4.4 SDV8 Autobiography

Gramophone

In my humble opinion, (and i'm very critical on sound) it very much boils down to the recording quality at source. I have tracks on ipod which sound far better than CD and vice versa. Furthermore, bass and treble often need adjusting accordingly if you're that way inclined.

That's a very interesting observation, roberlar.

I confess I have been going through a bit of mental anguish. I recently ordered the 1700w, for convenience I will use iPhone and preferably over bluetooth but I wouldn't die in a ditch plugging it in. I didn't specify the CD player. I have tried Tidal but for everything else (basically Bang & Olufson earphones and B&O A8 or something hifi speakers, so not the highest end output) I could not tell the diff between Master and Apple Music. Shoot me. BUT I CAN massively tell the difference between my last car's hifi and what will be my new car's hifi, and it's amazing.... which takes me right back to notty's observation: does it sound right to you?? Answer: yes

So maybe I will not be chasing what is supposed to be the best quality but instead I will be happy if it just sounds punchy and delicious to my old ears (not too old to appreciate Ruff Sqwad's Functions on the Low!). Any audiophile friends can buy their own car.  L405 P400e Autobiography (MY2020)... Silicon Silver / Espresso