My personal lived, yet anecdotal experience is with a fairly decent Kenwood head unit in my car. I'll often use bluetooth for convenience but from time to time

My personal lived, yet anecdotal experience is with a fairly decent Kenwood head unit in my car. I'll often use bluetooth for convenience but from time to time I'll connect over USB. Whenever I make the direct USB connection the sound quality is always noticeably better: rounder deeper sounds, much better quiet and loud sounds ... This leads me to conclude that one or other of the following must be the case: - The iphone DAC is x10^9s better than the Kenwood DAC - Kenwood and iPhone have negotiated a poor codec > most likely the problem is careless software/firmware So, don't take this wrong, but I'm going to wince and say this has a shade of a no true scotsman argument to it ... and I say this because I think that this is crux of what's wrong with Bluetooth. It is a very closed technology, and it's very hard to get a leg-up on the standardisation or how to use it for your own devices if you want to do anything any way commercial. I'm saying this as a Telecomms guy, who is used to having high quality standards documentation, and reference implementations for just about everything I do. Interoperability is key. Though Bluetooth is a communications standard, it seems to have been influenced more by consumer electronics than comms. After 10-15 years or so of using Bluetooth, it increasingly feels like a technology not really developed in good faith. You get these clunky imprecise results that I and other people report all the time. Working with a bluetooth device you're always going to be throwing the dice vs the certainty of plain old wired connections. It's plain to see that right across the industry people are hedging against it, and anybody that does require reliable M2M short-wave radio is using a proprietary protocol. Bluetooth is a millstone around our necks. Yes it gives you freedom from wires and a certain limited amount of interoperability, but you don't get a huge amount that you wouldn't have got vs proprietary radio technologies.