Originally Posted by Lifted07Duramax
Out of the 6 options you gave which ones would have the best sound quality and which ones would be the worse?
The Combox options give you maximum quality sound and would be the most convenient (once you have the Combox) since you also have the option for Bluetooth Audio streaming with that, but it's definitely the most expensive option. I don't know exactly how much a retrofit is, but if you PM a vendor like Mike Benvo I'm sure he'd be happy to tell you.
Importing media to iDrive would also give you maximum quality sound since your car gets digital audio that way just as it would with Combox, but it's far less convenient both to get the music in (have to put it on a flash drive, can't just get it from your iPhone) and navigating your music going forward (no playlist functionality, etc).
After that, the Y-cable would be the next best option in terms of quality, and it's also very convenient and a very low-cost option.
Next step down would be the Bluetooth gateway. Bluetooth Audio does sound very good, but if you have even somewhat high-quality audio files (160+ Kbps MP3s or 128+ Kbps AAC), you'll probably be able to tell a difference between that and a direct connection, even an analog Y-cable one. Bluetooth Audio only has limited bandwidth so audio often has to get downsampled to stream that way, and the gateway itself will still be connecting to your car via an analog connection after the downsampling, which is why I think a Y-cable would sound better. That said, I personally use Bluetooth Audio the majority of the time because I hate having to connect my phone every time I get in the car and I also tend to forget to bring it with me when I arrive where I'm going -- but on longer trips I'll connect it to enjoy somewhat fuller sound. The gateway would probably also be more expensive than getting a Y-cable plus the Lightning to dock connector, but I'm not certain about that.
And the worst option (if it even works) would be to just add a headphone cable to the cable you've got now and run it from your iPhone's headphone jack to the car's AUX in.