CD vs USB vs Bluetooth vs AUX
In the past week of Crosstrek ownership, I have been playing around a lot with the different inputs available on the stock, non-nav, stereo head unit. Everything I did was just based off of opinion of what I liked better, nothing technical about it at all.
What I found is that, for me, Bluetooth Audio is the way to go. CD and USB both sounded better overall -- deeper bass, cleaner mids, crisper trebel -- but there were problems with using both.
With CDs it comes down to variety for me. I listen to a lot of types of music and having a one-disc CD changer is just not convenient for someone who listens to something different about every time I sit in the car. A six-disc changer would be another story as not only do CDs sound great ("great" being relative to the other inputs), but they are also easy to navigate through songs with both the dash and steering wheel controls.
USB has two great qualities going for it: the sound was on par with CDs and with a 32gb+ USB drive you have the option of a huge variety of music available to you. However, the navigation of a USB device is absolutely dismal. Even if the sound quality was twice as good as anything else it would take a lot of convincing for me to use USB for audio. When navigating, you can only view ONE folder at a time so you have to scroll one-by-one to find what you're looking for. Secondly, it doesn't recognize folder structure. Let's say you have two Beatles albums, Abbey Road and Rubber Soul. Instead of having a folder labeled 'Beatles' with the two albums inside that you could navigate to, the Crosstrek USB interface reads it as three different folders, not two folders inside a master artist folder. This means that every album you put on the USB device will be seen as its own folder and you would have to scroll through them all, rather than just artist folders, to find what you're looking for. This makes it extremely difficult if you aren't sure exactly what an album is titled or if you just want to listen to the entire discography of a certain artist. That, along with the excruciatingly slow navigation (i.e. scrolling folder by folder and waiting for it to load the folder title) make USB unusable for me.
Though CD and USB sound best, AUX and Bluetooth don't sound bad and with the convenience of having a large selection of music along with a navigable interface (on your phone/MP3 player), either one of these is definitely the way to go IMO. I prefer Bluetooth just because all of my music is on my phone so I don't have to plug anything in -- I just get in and the phone connects with the car and starts playing whatever I want.
Overall I guess it comes down to what you're looking for and different sources will work for different people. That said, don't get me started on how bad the radio sounds...
I'd like to hear everyone else's opinions and preferences!