I have been planning to buy the hd 598 for quite a while because of all the positive reviews, but just recently, i noticed the q701. I plan on buying a pro-ject headbox s and usb box s (or odac). I listen mostly to rock, pop, country, and jazz. I also have the majority of my library ripped from...

I was wondering, if I have a usb dac that is connected to an amp, does it not get matter if I have onboard audio? In other words, does the "garbage in, garbage out" theory not apply if I use USB?