Bluetooth audio doesn't sound significantly different than CD to me. USB to iphone sounds the best, but acoustically I'd like to see some data to back this up. Bluetooth is very well researched and implemented as an xfer medium as far as wireless goes.
I do music production and mastering as an aside, my ears aren't legendary by any means but I make it a point to have high fidelity, beautiful sounds in my life every day (Yamaha HS80s, NI, Event 8 inches, fully sound-staged and trapped master room.. tons of DSP, tube processing racks, etc.. I love music.
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That being said, the 2013 paper cone system is totally junk in both BRZ and FRS models.. updated pio headsets are a huge step up. Bluetooth music might sound terrible to you because of an improperly set up sound stage combined with a huge disparity in your low-end (read: you're overpowering the already fatiguing stock setup with your dual 12's)
1. What's your drivers and amp specs?
2. Does the alternator kick in for every large electrical load/"sub notes"- or just intermittently?