Usb is great, but you can have crappy converting still. Digital is still converted. Think of accents. Yeah we all speak American but a New Yorker can't understand a guy from the brush of Mississippi. Digital is the same way. Is all digital but it had to get converted for each part to understand it. A lot of times one simple digital to analog conversation is best. So go great with the dac's and Bluetooth adapters or don't get caught up on what could be.
Technically the only one D/A conversion could be done by PXA. Bluetooth is a digital streaming, the adapter convert digital to digital (optical) then the signal go into DSP. I hope so... anyway I have a Clarion with digital output and I am going to compare it.